Which amongst the following is not a polyhedron?

Option (a) is a polyhedron because it satisfies its definition which is the solid is made up of polygonal region called faces and these faces meet at edges called line segments and the edges meet at vertices are called point.

(b) is a polyhedron because it satisfies its definition which is the solid is made up of polygonal region called faces and these faces meet at edges called line segments and the edges meet at vertices are called point.

(c) is not a polyhedron because it does not satisfy its definition. The figure contains circular base.

(d) is a polyhedron because it satisfies its definition which is the solid is made up of polygonal region called faces and these faces meet at edges called line segments and the edges meet at vertices are called point.