Ocean ridges generally are located:
a. near the continental rise.
b. at submarine canyons.
c. at spreading centers.
d. near ocean trenches.

Ocean ridges are usually located at divergent boundaries between tectonic plates, which is where two plates are moving away from each other. So your correct answer is C, at spreading centers.

Option (C)

Explanation:

In a divergent plate motion, when two plates moves in the opposite direction, the sea floor spreading takes place. As a result of which the lithosphere is stretched and becomes thin. The magma eventually rises up forming a ridge which is known as the mid-oceanic ridge. for example, the Mid-Atlantic ridge.

The rocks that are formed along this mid-oceanic ridge are the youngest and the rocks that away from the  id oceanic ridge are the oldest rocks.

Hence, the correct answer is option (C).