Any non-planar graph has K5 or K3,3 as a subgraph. Both these subgraphs are non-planar. K5 has 5 vertices and 10 edges. K3,3 has 6 vertices and 9 edges. Since we are interested in the non-planar with the fewest number of edges, option (B) is the correct choice.
Which of the following graphs has an Eulerian circuit?