Question

Hey can someone help with my simple englisb quesion.

Answer 1

C. Adverbial

The underlined word express where is the boat, so it's an adverbial.

Answer 2

The correct option is C, adverbial. "At the end of the dock" is indicating location. The deep structure would read "the boat that is at the end of the dock is...". The phrase  is a prepositional phrase, whose head is the preposition "at", but it is functioning as an adverb.