Are formed when one crustal plate is forced under another.
a. rift zones
c. mid-ocean ridges
b. trenches
d. continental shelves

The correct answer to the question above is (b.) trenches. Trenches are formed when one crustal plate is forced under another. It often occurs at an underwater subduction zone. It is generally deeper than it is wide. 

Trenches are formed when one crustal plate is forced under another.

Geothermic gradient is the rate of increasing temperature with respect to increasing the depth in the earths interior. away from tech tonic plates boundaries, it is about 25-40 c/km (72-87 degrees f/mi) of depth near the surface in most of the world.

I'm pretty sure it's c. extremely gradual but not 100% sure though.