A graph is bipartite graph when it can be coloured using 2 colour such that no two adjacent vertex are of same colour.
For a- Cn will be bipartite when n is even as then starting at any vertex and colouring alternatively will satisfy above criteria. But when n is odd then we require 3 colours and thus is not bipartite.
For b- As Wn is a wheel one vertex is connected to all in a cycle thus we need more than 2 colour which voilates the criteria for bipartite graph.