The correct answer is B. Predicate adjective
Step-by-step explanation:
In grammar, a subject complement or predicate complement refers to the word or words that go after linking verbs including verb to be and that completes the idea and defines the subject. Additionally, this predicate or complement can be a predicate noun in the case the words that complete the idea are nouns; a predicate pronoun in the case of using adjectives to define or complement the subject of a predicate pronoun in the case of using a pronoun. This implies, the sentence "Harte’s stories of frontier life were very popular" that contains the linking verb "were" and a in which the predicate is "very popular" contains mainly a predicate adjective because the subject "Harte's stories of frontier life" is defined and complement using two adjectives rather than nouns or pronouns.