a face.
It is called a face.
cone
The answer is a cone
a face
A conic section.
a pyramid
It can be a sphere, a spheroid, an ellipsoid, etc.
A polygon is a many sided 2-dimensional figure, and thus could never be "solid" Alternatively, it could be "one face that is a polygon". In that case the solid could be like a pyramid that morphs into a cone or a part-sphere. The base would be the one polygon, the other face would not.
A square pyramid
It is a sphere.
triangular prism