According to the theory of plate tectonics,

a the earth’s crust is in motion due to sea floor spreading that occurs
b the earth’s crust is stationary and the continental land masses have always been
where they are currently
c the earth’s crust used to be completed covered by water
d the earth’s crust covered the whole planet with barely any water

Answer;

A. the Earth’s crust is in motion due to sea floor spreading that occurs

Explanation;According to the theory of plate tectonics, the Earth’s crust is in motion due to sea floor spreading that occurs. The theory explains the movement of earth's surface and the features of the earth's surface.Plate tectonics theory is a theory that indicates that the outer shell of the earth is divided into several plates that glide over earth's mantle.The theory of plate tectonics merges the idea of continental drift and that of seafloor spreading.The convection in the mantle is the driving force which makes the plates to move. The convection drives plates tectonics through a combination of pushing and spreading at mid-ocean ridges.