For a graph with 2 vertices, the number of edges is 1 and degree of each edge is 1.
Value of option A, B, C = 1
Value of option D = 1/2
So D cannot be the answer.
For a graph with 3 vertices and 3 edges, degree of each vertex is 2.
Value of option A, B = 1
Value of option C = 2
So A and B cannot be the answer.
Therefore C should the answer.
(My answer doesn't match with the given solution, but I arrived at this conclusion by eliminating the options.)