"turning to fish as a cheap and healthy alternative"
Solution 1:
I strongly believe you are correct, and the IELTS have made an error (To err is human!). I think you should contact them and ask for an explanation of their answer. Examiners & organisations do make mistakes! Here is my reasoning as to why B cannot be correct.
B. Fish is not cheaper than meat
This statement is in the present tense.
Here is the original statement...
People have been turning to fish as a cheap and healthy alternative to meat
This uses the present perfect progressive tense, which indicates the condition is ongoing into the present (and possibly the future)! Here's a definition...
Present perfect progressive tense describes an action that began in the past, continues in the present, and may continue into the future. This tense is formed by using has/have been and the present participle of the verb (the verb form ending in -ing).
Thus, B cannot be correct, fish MUST be cheaper than meat, and your answer C is the only one left unfalsified.
C. Fish used to be seen as a cheap meal.
Good luck challenging the IELTS! Right is on your side!
Solution 2:
Let's eliminate as many options as possible
…where fish was once considered as food for the poor rather than the rich.
This excerpt excludes A) fish used to be seen as a luxury
Food scares like BSE and foot-and-mouth disease have also driven people away from eating meat
That clearly contradicts D) food scares have driven people away from eating fish
We have B) and C) left.
People have been turning to fish as a cheap and healthy alternative to meat, driving up demand and depleting stocks.
That fish was seen to be less expensive was true in the past but today the cost of fish has risen due to its high demand. In the excerpt, "a healthy and cheap alternative to meat" does not imply fish is cheaper than meat. Thus, B) fish is not cheaper than meat is the correct answer.
EDIT
C) fish used to be seen as a cheap meal
Answer C employs, what is called a semi-auxiliary verb, used. It is used to describe habits and habitual actions in the past. The British Council's page: Past habit – used to/would/past simple explains its meaning and usage very clearly: “Remember that ‘used to’ is only for past states/actions that don’t happen now – we can’t use it for things that still happen now”. If anything, the statement is reflected in the sentence:
…where fish was once considered as food for the poor rather than the rich.
Consequently, option C is not incorrect but it is not the main message of the speaker. By stating “In the UK fish used to be seen as a cheap meal” the candidate is ignoring the main point being discussed, that is, fish supplies are running low and their price is steadily increasing. I believe the examiners (or authors) were looking for a line that summarised that particular passage. If fish was cheaper in the past, it stands to reason that it is no longer true in the present.
Therefore, B) appears to be the better answer, but only just.
Solution 3:
As I mention at the bottom of this answer, I think this is a poor question that is unnecessarily confusing, but the likely justification for designating the answer as B is easier to see when observing the passage in its entirety.
I think it's important to listen to the entire recording in this case, and not just the portion cited in the question. Although the portion cited is the part that references "The UK" in particular, the message of the entire passage is more clearly focused on the increased demand for and scarcity of fish since the 1970's, so the primary message is, as Mari-Lou A's answer details, the focus on scarcity, and hence that fish has become more expensive than it was before.
In this transcription of the complete passage, I highlighted in bold portions that I think are especially relevant.
I would like to focus on the current problems in the fishing industry in Europe, and in particular the present scarcity of marine fish...
A statistic to begin with: Since the 1970's, stocks of the most heavily fished species have fallen on average by 90%. And why has this happened? Well, there's a chain of events that begins with the demographic changes that have taken place in the world over the last century. During this time, the world population has grown at a phenomenal rate, with efficient and heavy fishing which is technology driven meeting the increasing demands for food. ... as a consequence, many fishing stocks in European waters... are now on the verge of collapse. But the problem is not restricted to European waters. It's a situation that's all too clear all around the world. Fish stocks in the Pacific Ocean, for instance, are now on the verge of collapse, due to a combination of over-fishing and natural changes in Ocean ecology. And there's another reason behind the increased demand for fish, and that is the changes in eating patterns of different countries. Certain countries have a long tradition of fishing, for example, the Southern European countries.
And the portion already cited:
But eating patterns have changed in countries like the UK, where fish was once considered as food for the poor rather than the rich. People have been turning to fish as a cheap and healthy alternative to meat, driving up demand and depleting stocks. Food scares like BSE and foot-and-mouth disease have also driven people away from eating meat, which again is invariably replaced by fish.
However, having made this point, I also believe that this is a poorly written and confusing question, specifically because of the prompt "In the UK". This line draws attention to the final portion of the text, which does mention that one reason people have turned to fish is because it is a "cheap and healthy" alternative to meat. There's no question that this makes the prompt confusing when it asks specifically about the status of fish In the UK.
So my conclusion is that it is a poor question, but the reason the answer is designated B is likely because of the overarching message of the passage is focused on scarcity of fishing stocks, increased demand for fish, and thus that fish is becoming more expensive.