The correct answer is c.) The verb expressing a state of being is called a linking verb because it connects—links—the subject of a sentence with a word or a word group in the predicate part of the sentence.
Linking verbs, also known as copular verbs, are used to link the subject of a sentence with a word or a word group in the predicate that describes or identifies it. These verbs do not show action, but rather they express a state of being or condition. Examples of linking verbs include "is," "am," "are," "was," "were," "seem," "become," and "appear."
The term "linking verb" comes from the fact that these verbs act as a link between the subject and the predicate, connecting them in a sentence.