The answer is WHO.
Who is a pronoun that refers to the subject of the sentence, meaning that it is the one that directly interacts with the main verb, or performs the main action, Therefore, when answering questions with "who", the answer may be expressed with subject pronouns, namely, I, we, you, he, she, it, they.
Thus, "Who will be answering students questions at the job fair?" is correct because "who" is asking about the person that will perform the main action "answer questions", that is to say, the subject. Furthermore, when answering this question, we may use a subject pronoun, for example, "They will be answering students questions at the job fair".