There is only one perfect matching possible for this graph which you mentioned in the question itself. {ab, ce, df}. This is the only set which represents perfect matching. Hence answer is 1. If two sets could represent the perfect matching for this graph, answer would be 2. Hope this helps.

perfect matching is a matching that is used to cover every vertex in a graph

here edges set {ab,ce,df} form the perfect matching

here only one set of perfect matching is possible because we can not make more set like this which cover all the vertices but no vertices repeat (means every vertices exactly onces )