Answer:
Regular Pentagon
Explanation:
A regular icosahedron is a twenty-faced polyhedron, each face being an equiangular triangle. Each vertex is joined together by 5 faces, therefore the polygon formed at each vertex is a regular pentagon.
We can also figure out the number of faces at each vertex using Euler's formula
F+V=E+2
F=number of faces = 20
E=number of edges = number of triangular faces * edges/triangle /2
(since each edge is shared between two faces)
= 20*3/2
=30
Number of vertices
= E+2-F = 30+2-20 = 12
So number of edges meeting at each vertex
= 30 / (12/2) = 30/6 = 5
(12/2 because each edge joins two vertices).
See attached figure, courtesy of Wikipedia.