Asimple subject can be either a noun or a/an
a. verb.
b. adjective.
c. adverb.
d. pronoun.

The answer to this problem would be D. pronoun.

Verbs are considered to be the perfect bridges and connectors of the subject and the predicate. Adjectives and adverbs would be the spices of the sentences which would pave way to a much better understanding of what the sentence is talking about, giving the reader or listener to have a clearer picture rather than the usual simple subject and simple predicate. Pronouns, on the other hand, act as substitutes for nouns and are usually used when the noun is overused in a paragraph. This is to avoid redundancy and the monotonic feel when read or listened to.