The correct answer is - b. seafloor spreading.
The divergent boundaries are plate boundaries where the tectonic plates are moving away from one another. As they move away they live much thinner crust with lot of cracks behind them. The high pressure from the mantle easily manages to push upwards the magma at these places. The magma reaches the seafloor and cools off very quickly, creating new ocean crust. As more and more magma comes out and more and more new crust is formed, the seafloor is pushed aside and it is spreading.
