No. of vertices in the given questions $= n$
In an undirected simple graph, the maximum no. of edges that are possible $= n(n-1)/2$
Now, each edge can be either present or absent in a graph. So, there are 2 possibilities for each vertices.
Therefore, total no. of possible graphs $= 2*2*2*... n(n-1)/2$ times
=$2^{n(n-1)/2}$
So, correct answer is option no. D.