The only platonic solid with less than five faces is which of the following
a. icosahedron
b. hexahedron
c. tetrahedron
d. octahedron

Option: C is the correct answer.

                C.  Tetrahedron.

Platonic solid--

Platonic solid is a convex polyhedron which is constructed with the help of repetition of regular convex polygonal faces.

There are 5 types of platonic solid:

 1) Tetrahedron--

It is a polyhedron which is composed of four triangular faces.

  2)   Cube--

It is a polyhedron which is constructed with the help of square faces and has 6 faces.

 3)  Octahedron--

In geometry a octahedron is a polyhedron with 8 faces.

Each of the face is composed of equilateral triangles.

 4)   Dodecahedron--

It is a polyhedron which consist of 12 faces.

 5)  Icosahedron--

 It is composed of 20 faces.

Hence, the only Platonic solid with less than five faces is :

                C.   Tetrahedron

(a) the volume of the cone is 20/3 in3

hence, option a is correct.