In the question “Who’s that?”, the word that stands in place of a person or object and functions as a Demonstrative Pronoun.

১. It is not an adjective because it doesn’t modify a noun following it (e.g., that boy).
২. Adverbs modify verbs or adjectives, and conjunctions connect clauses, which ‘that’ does not do here.
৩. Pronouns are words used instead of nouns to avoid repetition.

অতিরিক্ত তথ্য: This, that, these, and those are common demonstrative pronouns.