Ocean ridges form as a result of
a.
slab pull
b.
the upwelling of magma from the asthenosphere
c.
sediment being deposited from the ocean
d.
all of the above

The correct answer is - b. the upwelling of magma from the astenosphere.

The ocean ridges form on the boundaries between two tectonic plates, or more specifically, on the divergent boundaries that are located in the ocean. They are essentially a very long underwater mountain ranges.

When there's a divergent boundary between the tectonic plates, the plates are moving away from each other, thus making the crust on the separation line much thinner. Because the crust is getting thinner, the magma from the mantle layer manages to push through it, and as it does that it quickly cools of in the water and makes new crust, that initially is manifested as an ocean ridge.