Answer
The answer is a pentagonal polyhedron. Escher's work mainly featured Platonic solids (tetrahedron, cube, octahedron, dodecahedron, icosahedron) and Archimedean solids (truncated octahedron, truncated icosahedron, rhombicuboctahedron, etc.). While pentagons appear in Escher's art as part of tilings or regular divisions of the plane, he did not depict pentagonal polyhedra as frequently as he did other polyhedron.