What form the bases of a prism?

The faces are always rectangles on prisms, so if it's a triangular prism the bases are triangles; if it's a rectangular prism, the bases are rectangles; if it's a hexagonal prism, the bases are hexagons.

Any polygon can form the base of a prism.

Some people also consider cylinders to be circular prisms. In which case any closed plane shape can form the base of a prism.