Reference string: 0 5 9 13 7 0 15 25
0 - set 0 - Compulsory miss
5 - set 1 - Compulsory miss
9 - set 1 - Compulsory miss (set 1 now full)
13 - set 1 - Compulsory miss (5 replaced, but this is not conflict miss; if 5 is again accessed then that access causes conflict miss by its definition. Remember that conflict miss is something which can be avoided if the cache is fully associative and hence it can never happen with compulsory miss)
7 - set 3 - Compulsory miss
0 - set 0 - Hit
15 - set 3 - Compulsory miss
25 - set 1 - Compulsory miss - 9 replaced. Again this is not a conflict miss.
So, number of conflict misses = 0.
Ref GATE question.