Question

ocean trenches are deep canyons on the ocean floor. remember that earths crust is made of tectonic plates that move and can bump into one another. trenches are formed when plates hit each other. one plate sinks under another and melts into earths mantle. this process is called subduction. the deepest place on earth, the challenger deep, is found inside the mariana trench in the pacific ocean. you could put mount everest inside it and still have a mile left on top!
ocean trenches are formed by _.
a tectonic plates colliding
b mountains moving into canyons
c earths crust freezing into a solid
d ocean water freezing into ice

Explanation:

The text states that Earth's crust has tectonic plates that move and when they hit each other, one plate sinks under another (subduction) and ocean trenches are formed. So they are formed by tectonic - plates colliding.

Answer:

A. tectonic plates colliding