Your assessment of parts (a) and (b) is correct. It sounds like you need some additional tools to describe the differences.
One tool is degree sequence, which simply lists the various degrees of the vertices in sorted order. If two graphs have a different degree sequence, they are not isomorphic. However two graphs that have the same degree sequence may or may not be isomorphic.
The only example here is the right-hand graph in part (a) has a different degree sequence to its counterparts, since it has no degree-3 vertex.
You can also check which vertex degrees are adjacent. So there is vertex of degree 3 in the left and centre graphs of part (a), but their adjacent vertices have degrees (2,2,1) and (2,1,1) respectively, so they are not isomorphic.
You can use similar tools to identify the graphs of part (b) as non-isomorphic.
In part (c), the graphs are all cubic (every vertex is degree 3) so you will need to think about the cycles that the vertices are part of, in particular the shortest cycle for each vertex. The right-hand graph is the famous Petersen graph.
At this point you may need to produce an explicit isomorphism. This consists of labelling the vertices and producing a mapping such that the corresponding vertices are connected in both graphs.