Can you guess what one of his tasty ingredients were?
Speaking of the alleged inventor of the hamburger, someone asked me
Can you guess what one of his tasty ingredients were?
It is understandable, particularly in context, but it still feels wrong. Shouldn't the final word be "was" since the sentence is about one ingredient out of many? Or is this acceptable due to the plural ingredients we are choosing from?
Is there anything else wrong with this sentence?
If the question were:
Can you guess what two of his tasty ingredients were?
then "were" would be correct. However, since only one ingredient is called for in the response, "was" would be the correct word.
"Ingredients" is the object of the preposition "of", while "one" is the subject of the relative clause. The rule is that the verb agrees with the subject. Therefore, "what one ... was".
Shouldn't the final word be "was" since the sentence is about one ingredient out of many?
Yes.
Or is this acceptable due to the plural ingredients we are choosing from?
No, but there's a reason why it happened, and why you say in a comment that correct "… ingredients was." doesn't feel comfortable to you either.
The basics of number here you clearly get or you wouldn't be asking this question, "The ingredient was…", "The ingredients were…"
But consider:
Chocolates or cake are available after the burgers.
Chocolates or cake is available after the burgers.
Now, these two sentences are more contentious.
Some would favour the first, on the grounds that the plurality of chocolates makes the expression chocolates or cakes plural.
Some would favour the second, on the grounds of the proximity principle in which the verb agrees with the closest noun, and hence the singularity of cake makes cake singular.
This is not a clearest "rule" in English (I hesitate to say "rule" at all). There are some cases where it clearly doesn't apply (e.g. "Alice and Bob" are clearly plural, as much as "Alice" and "Bob" are individually singular), and therefore when one should use it, if one should use it at all, is a matter of considerable disagreement.
And your example is not one such case, but it does show the psychological mechanism behind it at work.
But to the extent that the proximity principle works, it works because we expect to hear verbs agreeing with the closest noun, probably on the basis that most of the time that just happens naturally.
And your discomfort with either form demonstrates that.
You dislike the incorrect form, because you correctly detect the grammar mistake, and so it jars.
You dislike the correct form, because you expect to hear verbs agreeing with the nearest noun, unless the reason they do not is particularly obvious.
And therefore, even though you know "the sentence is about one ingredient out of many", you find yourself on EL&U, seeking the "correct" answer, though you had it already.