Trenches form at sites where
a. one plate descends beneath another.
b. erosion cuts into the continental shelf.
c. two plates diverge under the sea.
d. the tops of undersea volcanoes collapse.

The correct answer is (a.) one plate descends beneath another. Trenches form at sites where one plate descends beneath another. A trench is more like an excavation in the ground that is generally deeper than it is wide. It is generally used in battle formation, in which the front line is made of trenches.

Ithink the answer to that would be b):