I see students debating on this question. So let’s examine it together.
The question is what type of crust the denser plates are made of? The passage has no information about the type of material for the denser plates. The only clue is that denser plats that typically subduct at convergent boundaries relate to the deep sea .
So A is possible.
B is not good since continental crust is about land.
C is not good since mantle material is not mentioned.
D is not right since mid-ocean ridges happens at the divergent boundaries,.
So even though A is not mentioned, A is the best choice.
