The correct option is D Preposition
Here, 'on' as preposition is correct because it is used to show that somebody belongs to a group or an organization. Here, it is the committee.
Conjunctions are used to connect phrases, clauses, and sentences. 'On' is not a conjunction.
A pronoun is a word that replaces a noun in a sentence. 'On' is not a pronoun.
interjection is a part of speech which is more commonly used in informal language than in formal writing or speech. Basically, the function of interjections is to express emotions or sudden bursts of feelings. 'On is not an interjection.