* Subduction Zones: When two tectonic plates collide, and one is denser than the other, the denser plate (usually oceanic) is forced beneath the less dense plate (either oceanic or continental). This process is called subduction, and the boundary between the two plates is called a subduction zone.

Let's look at the other options:

* a. an ocean basin: Ocean basins are large depressions on the Earth's surface filled with water. While subduction can contribute to the formation of ocean basins over long periods, it's not the direct result of one plate going under another.

* b. ridge: Mid-ocean ridges are underwater mountain ranges where new oceanic crust is created as tectonic plates move apart. This is the opposite of subduction.

* d. rift valley: Rift valleys form when tectonic plates pull apart, creating a depression. This is also the opposite of subduction.