Where trenches do not exist, the steep continental slope merges into a more gradual incline known as the continental a abyss b zone c rift d rise

Hey there mate!

This would actually be known as a continental rise. If the trenches were to be (existed), then most likely steep continental slope merges into a more gradual incline known as the continental would rift. 

It's practically the opposite of each other, which is pretty neat in this way.

Where trenches do not exist, the steep continental slope merges into a more gradual incline known as the continental rise.

Your correct answer would be (option D).

I hope this helps you!