Answer:
D. Linking verb
Step-by-step explanation:
Verbs are words that express actions, occurrences, and states of being.
Transitive verbs are verbs that require an object (the recipient of the action expressed by the verb). To become is not a transitive verb as it can't have an object. This is why it is an intransitive verb used as a linking verb. This is its main purpose, which is why option D is the one that best describes it.
Linking verbs are verbs that provide additional information about the subject by connecting it to a predicate adjective or predicate noun. Just like the verbs to be and to seem, to become is a true linking verb. In the given case, Frank is the subject of the second independent clause, and the words that describe him are class president. These words are a predicate noun (predicate nominative). Became connects these parts of the clause, which is why it's a linking verb.