Question

Your mentor asks you to determine if the mutants belong to the same complementation group. what is true about flies that belong to the same complementation group?

Answer 1

This is true about flies that belong to the same complementation group:
They all have some mutation in the same wing-development gene.
Each strain may have a different mutation, but the same gene is mutated in all strains in a complementation group.

Answer 2

The flies all carry a mutation in the same wing-development gene. 

Each strain may have a different mutation but overall the same gene is mutated in the strains.