Ocean trenches are most often formed when A)two continental plates slide past each other.
B)an oceanic plate goes under a continental plate.
C)two oceanic plates grind past each other along transform faults.
D)two oceanic plates hit each other causing both plates to rise up.

B) An oceanic plate goes under a continental plate.

Explanation:

All of these are technically not the correct answer, the real answer would be something on the lines of: "An old dense plate pushes a lighter weight plate causing an ocean trench." But since that isn't an option, the best one would be an oceanic plate that goes under a continental plate.