With a rectangular prism it doesn't matter which face you say is the base, your answer will turn out the same.
Let's say the bottom face is the base (the one it's sitting on):
With a triangular prism, the bases are the parallel sides (where the triangles are).
The base of this prism is a trapezoid.
A cylinder is pretty much a prism since it has parallel bases. In this example the diameter of the circle is given. As we discussed earlier, the radius is half the diameter.