A regular graph is a graph in which all the vertices have same degree....yes obviously complete graph is (n-1)regular graph.
here in the question it is given Km,n i.e it is a complete bipartite graph but it is not true that for ny value of m,n it will be a regular bipartite graph...let us take K2,3 ..i.e m=2 and n=3 now it will be degree having 3,3,2,2,2.
If we take k 2,2 (m=n)now the degree will be 2,2,2,2