I'm not too familiar with the more advanced graph theory, but just by looking at Wikipedia (a) seems to be straight forward. Let G' be the complement of G.
G is isomorphic to G' iff there exists a bijective function f: V(G)->V(G') such that for any two adjacent vertices a,b in G, f(a), f(b) are adjacent.
Let f := the function that sends:
a -> d
b -> c
c -> a
d -> b
Clearly f is bijective.
Also:
(a,c) is an edge in G, and (d,a) is an edge in G'
(a,b) is an edge in G, and (d,c) is an edge in G'
(b,d) is an edge in G, and (c,b) is an edge in G'
So f is a graph isomorphism.
