The correct answer is (d.) seamount. A seamount is a mountain rising from an ocean floor, however, it does not reach the water surface, thus, it is not considered as an island or whatsoever. It was indeed once a volcano under the ocean.
A Seamount is an underwater mountain that can rise hundreds or thousands of feet from the seafloor. Seamounts are generally extinct volcanoes that, while active, created piles of lava that sometimes break the ocean surface.
