Pronoun-antecedent agreement with indefinite pronouns

17a. Each of the applicants must submit his or her completed form to the interviewer.

17b. Each of the applicants must submit their completed forms to the interviewer.

My answer is: 17b

Nope, not this time.

The correct answer is a, because EACH is singular and is the antecedent. The pronoun must also be singular.