The man in blue quietly stepped up to the window and paid for his ticket with cash.
the adjective phrase in this sentence is
a. in blue.
b. up to the window.
c. for his ticket.
d. with cash.

Adjectives (and adjective phrases ) describe nouns: so we must select a phrase that desctribes a noun. The correct answer is " in blue" - it gives extra information about "The man". C and D are modifications of "paid" and B of "stepped".