What word is used in place of a noun?

The correct term for a word that is used in place of a noun is "pronoun." Pronouns serve as substitutes for nouns to avoid repetition and make sentences clearer and less cumbersome. For example, instead of saying "Sarah went to the store because Sarah needed milk," one could say, "Sarah went to the store because she needed milk," with "she" acting as the pronoun that replaces the noun "Sarah."

The other options do not fulfill this specific function: adjectives modify nouns rather than replace them, conjunctions connect clauses or sentences, and prepositions show relationships between nouns (or pronouns) and other words in a sentence. Hence, the distinguishing feature of pronouns as substitutes for nouns makes it the correct answer.