SAT question, pronoun "their" [closed]

There is absolutely nothing wrong with the original sentence.

There would also be absolutely nothing wrong with this alternative, which is probably what the examiners wanted:

John was one of the astronomers who devoted all his time to science.

As FumbleFingers pointed out, this is partly because some (archaically) believe that 'he' is the only suitable gender-neutral pronoun.

In this case I think there are reasons why one might pick one over the other, because it can emphasize whether it's John or some anonymous group of astronomers devoting their time to science.

It is grammatical.

John was one of several astronomers who devoted all their time to Science.

"their" refers to "several astronomers" and John was just one of them.

"Mary Ann is one of the girls who are going to run the marathon." "

I think the point is the sentence is about John. The fact that John is an astronomer is secondary. Therefor the pronoun should relate directly to John.

That being said, I've no problem with the sentence you were given either, since it seems to me that the sentence could equally be perceived to be about a life of science, which makes the astronomers more important since John is only a small part of the group that devote their life to science.

Seems like a silly point to grade people on unless you've specifically been told an order in which you should focus on nouns in sentences.