Marine geology is a subspecialty of

a. oceanography.

b. geology.

c. maritime science.

d. environmental science.

I would definitely exclude maritime science and environmental science: both are more focused on the life and interaction of various parts of ocean, and less on the form of the rocks.

It is in fact a sub-discipline of both geology and eceanography: it's a part of geology by the virtue of focusing on rocks, and it's part of oceanography by the virtue of focusing on the rocks on the ground of the oceans