Answer:
C: They all have some mutation in some wing-development gene. Each strain may have a different mutation in a different gene, but all strains within a complementation group have the same phenotype.
Step-by-step explanation:
From the situation of the question given, we can say that the true statement about flies that belong to different complementation groups is option C primarily because Strains existing in different complementation groups obviously would have mutations in different genes.