Answer:
The answer should be A. lithosphere.
Step-by-step explanation:
Firstly, let's look at the options:
C. and D, both parts of the Earth at the core, have almost nothing to do with the Earth's crust or tectonic plates except maybe having a passive effect on them.
B. mantle is where the lava that moves the tectonic plates circulates due to convection, which is the reason they move at all. But, it is not where the tectonic plates themselves are.
Therefore, finally, we have A as the answer.
(As a sidenote, the "lithosphere" means the Earth's crust.)
Hope this helped!