The correct answer is - divergent boundaries.
The new oceanic crust is forming on the places where there's a divergent tectonic boundary between two oceanic tectonic plates. When two oceanic tectonic plates are moving away from each other, the start to form a gap between them, thus making the crust thinner. Because the crust is getting thinner, the extreme temperatures, pressures, and magma, from the mantle manage to penetrate through it and reach the ocean floor. As they reach the ocean floor, the large amounts of magma, as they get out, are cooling of very quickly because of the water and create igneous rocks, thus creating the new oceanic crust.