For perfect matching the number of edges to be selected such that no two edge are adjacent to each other.
Hence number of ways to select the edges in complete graph with 6 vertices = 6∁2 * 4∁2 * 2∁2
Now for the set of edges selected the ordering isn’t important, hence 6∁2 * 4∁2 * 2∁2 /3!
answer is 15 option A