For any complete graph Kn with n nodes, different spanning trees possible is n(n-2)
So, for K4, its 4(4-2) = 16
For any Bipartite graph Km,n with m and n nodes, different spanning trees possible is m(n-1).n(m-1)
So, for K2,2 its 2(2-1).2(2-1) = 2.2 = 4
so answer is C