The answer would be:
D. When two tectonic plates move in opposite directions, it causes magma to rise, creating a new seafloor.
If you'd like to know why:
Seafloor spreading happens at mid-ocean ridges. This occurs when two plates move away from each other. They create what you call a divergent boundary. When these plates move apart, magma wells up from the middle, and is cooled by the water surrounding it, which creates the newer or younger seafloor.
The image below shows what happens: