The infinitive is functioning as the predicate nominative. "Plan" is the subject (with Joel's as a modifier for "plan"). Since "is" is a linking verb, it is a predicate nominative and not a direct object.
The correct answer is B. As a predicate nominative
Explanation:
A predicate nominative also called a subject complement is a type of sentence complement that is introduced after a linking verb (such as verb to be, seem, look and feel, which are verbs that do not have an object), complementing the subject of the sentence mainly as it renames the subject. Thus, considering in the sentence "This summer, Joel's plan is to travel" the main verb is a linking verb (Verb to be) there is not an object, but a predicate nominative or subject complement which is the infinitive (basic form of a verb) "to travel", also, as it is stated about a predicate s nominative it renames the subject which means "to travel" is the same than "Joel's plan". Thus, in this sentence, the infinitive "to travel" is a predicate nominative as it is connected with a linking verb and renames the subject which is "Joel's plan".
