Answer:
Whom
Step-by-step explanation:
While "whom" is a pronoun that refers to the subject of a sentence, "whom" refers to the object of a verb or a preposition, meaning that "whom" does not perform the main action, but, instead, it receives the action, or it can be found near a preposition such as "to" and "from".
In the sentence, the missing pronoun should refer to the student, and since the student is the object of the verb "selected" (because it is the one that is receiving the action and not performing it), the correct pronoun is "whom."