Answer:
a) Sea-floor spreading
Step-by-step explanation:
The divergent plate boundaries are the plate boundaries where the tectonic plates move away from one another. As they move in opposite direction, the plates live behind a gap of very thin cracked crust. The magma from the upper layer of the mantle, the astenosphere, manages to break through this fragile, thin crust and comes out on the surface, or rather on the ocean floor. As the magma comes out, it quickly cools off in the water, creating new igneous rocks, thus new crust. The continuous pilling up of new crust at the divergent boundary results in the formation of a mid-ocean ridge, which constantly rises and pushes aside the old crust.