Which word functions as the subject in a sentence?

The word "who" functions as the subject in a sentence because it is a pronoun used to refer to the person or people performing an action or being described in a clause. When "who" is used to ask a question or to introduce a clause, it stands in for a subject position, thus fulfilling the role of the subject. For example, in the question "Who is coming to the party?" "who" is the subject because it refers to the person who is performing the action of coming to the party.

In contrast, the other options serve different grammatical purposes. "Whom" is used as an object pronoun, referring to the person who is receiving the action rather than performing it. "Which," on the other hand, is typically used to introduce non-defining relative clauses or to ask questions about one or more items in a selection and does not function as a subject. "That" is often used to introduce restrictive clauses and can refer to things or people but also does not serve as a subject in the same manner as "who."