The aleutian islands extend westward from southern Alaska to form the northern boundary of the Pacific Ocean. These volcanic islands were formed by the nearby ...

A. subduction of a continental plate

(B. subduction of an oceanic plate) - answer but why is this the answer?

C. divergence of a continental plate

D. divergence of an oceanic plate

The correct answer is B

Subduction of a ocean plate.

Aleutian islands extend westward from southern Alaska to form the northern boundary of the Pacific Ocean when the Pacific plate gets subducted under North American Plate at a dip of 45 degrees. The pacific is an oceanic tectonic plate that lies beneath the Pacific Ocean. And thus the correct answer is option B