Correct choice is (a) 24
Explanation: A vertex colouring of a graph G = (V’,E’) with m colours is a mapping f:V’ -> {1,…,m} such that f(u)!=f(v) for every (u,v) belongs to E’. Since in worst case the graph can be complete, d+1 colours are necessary for graph containing vertices with degree at most ‘d’. So, the required answer is 24.