Regular Graph = Every graph has same no of edges incident / Degree of each vertex is same
Complete Bipartite graph Km,n is regular if & only if m = n.
So
A) & B) are both false.
Counter example for A) K 2,1
B) K 1,2
As A & B are false c) both a) and b) must be false. (Even you take both option together m = 1 & n =1 don't give you set of all Km,m regular graphs)
D) Is correct. When m = n , complete Bipartite graph is regular & It can be called as m regular graph.