A graph is self-complementary if it is isomorphic to its complement. For all self-complementary graphs on $n$ vertices, $n$ is
By definition, a self-complementary graph must have exactly half the total possible number of edges, i.e., edges for a self-complementary graph on vertices. Since must be divisible by , it follows that or .
Answer: D. Congruent to , or, .
Source: Self-Complementary Graph -- from Wolfram MathWorld