Ocean ridges are formed by
a. subduction zones
b. transform boundaries
c. convergent boundaries
d. divergent boundaries

A mid-ocean ridge

Explanation:

A mid-ocean ridge or mid-oceanic ridge is an underwater mountain range, formed by plate tectonics. This uplifting of the ocean floor occurs when convection currents rise in the mantle beneath the oceanic crust and create magma where two tectonic plates meet at a divergent boundary.

Ocean ridges are formed by divergent boundaries.

Option-D

Explanation:

Ocean Ridges are underwater physical features formed along the two divergent boundaries. When two divergent boundaries are adjacent to each other and they keep diverging, ocean ridges form as a result of increasing gap between them which ultimately results in the volcanic eruption which later spreads, cools down and forms as ridges under the oceanic surface while repetition of this process leads to the formation of large oceanic ridges.

It is often noticed that, the age of the ridges keeps on increasing as one moves from top to bottom.