Number of spanning trees:

For complete graph:

$\tau(K_n)=n^{n-2}$

A: $K_3 = 3^{1} = 3$

B: $K_4 = 4^{2} = 16$

For complete bipartite graph:

$\tau(K_{m,n}) = m^{n-1} n^{m-1}$

C: $K_{2,2} = 2^{1} \times 2^{1} = 4$

For cycle graph:

$\tau(C_n)=n$

D: $C_5 = 5$

Thus:

A = 3

C = 4

D = 5

B = 16

Increasing order:

A, C, D, B