The sentence in this set of sentences that has a problem with the pronoun-antecedent agreement is:
A student sometimes discovers he or she doesn't like their chosen field.
Explanation:
The pronouns are "he or she" and "their." These pronouns are preceded by the noun "student." A better construction of the sentence is the first one, which says: "Students sometimes discover they don't like their chosen field." The third sentence is also well-constructed. They are simpler and less complicated than the second sentence. Again, the first sentence has perfect agreement between the noun "students" and the pronouns "they" and "their."
