In which sentence is the predicate nominative correct A This is he B This is him C It is them D It is her?

The correct pronoun functioning as a predicate nominative is A he.

A predicate nominative is a noun or pronoun that follows a linking verb to restate the subject of the sentence.

A linking verb is a verb that acts as an equal sign, the subject is or becomes the object (this = he).

A pronoun functioning as a predicate nominative is always the subjective form.