4. A graph G has k isolated vertices and n + k vertices. The maximum number of edges graph G can have?

a) n(n-1)

b)n(n-1)/2)

c) n(n-k+1)/2

d) n(n+k-1)/2

Since isolated (degree 0) vertices can not have edges. So effectively we have only n vertices which can hold edges. form complete graph structure with that n vertices. So they can have n(n-1)/2 edges among them.