How does the infinitive function in the following sentence: This summer, Joel's plan is to travel.

A - as an adjective

B - as a predicate nominative

C - as a direct object

D - as a subject

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.