The question is very tricky, you don't need anything special here, you just need to revise the definition of CPI and when do CPI get equals to 1.
According to question even if you use renaming technique or any technique, there will be 3 stalls for sure.
But " successful characteristics of a pipeline is in every new cycle, new input must be inserted into a pipeline therefore cpi=1".
So you can see even if there are 3 stalls but in every new cycle one new instruction get inserted so the answer would be 5.