A graph is called a complete graph ($K_n)$ if each vertex is connected to the $n-1$ remaining vertices.
For eg $K_3$ graph having 3 vertices and 3 edges.
in the same way, $K_4$ having 4 vertices and 6 edges.
For $K_n$ graph with $n$ vertex, $\frac{n(n-1)}{2}$ edges should be there.
So with the given options, only option B is satisfied. option B is correct.