Answer:
A. At a divergent boundary in an oceanic plate
Explanation:
A rift valley is a lowland region formed between several mountain ranges which forms where the tectonic plates of the earth move away from each other, or "Rift". They occur entirely due to tectonic activity, and no erosion is involved, they can be found both on land and at the bottom of the ocean floor. At convergent plate boundaries, tectonic plates crash into each other resulting often in volcanoes, and formation of mountains, which is why answers B and D are incorrect.
And although rift valleys can occur both in continental and oceanic plates, most of Earth's deepest rift valleys are underwater, dividing long mountain ranges called mid-ocean ridges, and are the result of seafloor spreading. The crust that makes up continental plates is thicker but less dense than oceanic crust, due to the fact it's composed of lighter rocks and minerals; oceanic plates on the other hand are made up of heavier volcanic rock, which is the result of magma flows from mid-ocean ridges, and as tectonic plates move away from each other at mid-ocean ridges, magma from the mantle raises and hardens as it contacts the cold water, and that way forming new oceanic crust. There are very few active rift valleys found on continental plates compared to oceanic plates, that's why answer A is correct.