A simple subject can be either a noun or a/an

A. adjective.

B. adverb.

C. verb.

D. pronoun.

The correct answer would be option D. A simple subject can either be a noun or a pronoun. A pronoun is a word that can be used to replace a noun; therefore, it can function as a simple subject too. Examples of pronouns include: I, you, he, she, they, it and we. These are the pronouns that can be used as a subject.