This summer, Joel’s plan is to travel.

How does the infinitive function in the sentence?

A) as an adjective ***
B) as a predicate nominative

What does the infinitive modify?

I think it would either be Summer or Joel. I am leaning more towards summer. Wouldn't It be B then and not A.

Yes, it's A. The infinitive is a predicate nominative following the linking verb, "is." It means the same as "plan."

It is B

The infinitive "to travel" functions as an adjective in the sentence "This summer, Joel's plan is to travel." It is used to describe or modify the subject "Joel's plan." Specifically, it tells us what the plan is. The infinitive "to travel" answers the question of what Joel's plan is for the summer.