A pyramid is a solid figure in which the base is ________.

A pyramid is a solid figure, the base of which is a polygon.

The polygon that forms the base of a pyramid can be any type of polygon, such as a triangle, rectangle, square, or pentagon, among others. The shape of the base determines the type of pyramid it is. 

For example, if the base is a triangle, it is called a triangular pyramid or a tetrahedron; if the base is a square, it is called a square pyramid; and if the base is a pentagon, it is called a pentagonal pyramid.