ANSWER
D. a
EXPLANATION
A prism has two faces that are equal. These are called base and top (usually referred to as base as well), and their shape defines the name of the prism - for example, triangular prism or rectangular prism.
The other faces are all rectangles.
In this figure, we can see that b, c, and d are rectangular faces, so none of these is the base. Hence, a is the base of the prism.
