Answer: A. Any one of the flat surfaces of a prism or a pyramid.
Step-by-step explanation: In geometry, a face refers to a flat surface that bounds a three-dimensional solid. A solid can have multiple faces, and each face is a flat polygon. In the case of prisms and pyramids, the faces are specifically flat surfaces.
For example, if we take a rectangular prism, it has six faces - each of the four sides is a rectangle, and the top and bottom faces are also rectangles. Similarly, a pyramid has triangular faces that meet at a common vertex.