Which pronoun could function as a question word in a sentence?

The pronoun that functions as a question word in a sentence is "who." This pronoun is typically used to inquire about the identity of a person or people. For example, in the question "Who is coming to the party?" "who" directly asks for information regarding a person's identity.

In contrast, while "where," "that," and "which" may also play crucial roles in different contexts, they do not serve the same function as a question word regarding identifying individuals. "Where" asks about a location, often requiring a place as an answer. "That" is primarily a relative pronoun used to introduce a clause, while "which" typically functions to specify or differentiate among options, rather than directly soliciting identification in a question.