Since the ___, the company has doubled in size [closed]

Since the ___, the company has doubled in size.

a. 1990's

b. 1990s

c. 1990s'

d. 1990ies

After searching on Google, I think the answer is b. However, when I submit the final answer, it says that my answer is incorrect. I have been reading a lot of articles and I found that the authors are also using 1990s, 1970s, etc. Anyone can correct my understanding when mentioning the year? Thank you.

1990s is the way to go. There's no possession here. The only time when it's appropriate to use an apostrophe with digits is if the text would be confusing. (E.g. "1's and 0's" is clearer than "1s and 0s".)