Fruit Fly Species All Look More or Less Alike

Fruit fly species all look more or less alike. If you have a male and female fruit fly, what would be the best way to determine that they are the same species?

A) If they can both asexually reproduce and their offspring can also successfully asexually reproduce, then they are all the same species.
B) Examine them closely with a low-power microscope, comparing their physical characteristics to published species key lists of characteristics.
C) Base sequence the DNA of their chromosomes.
D) If they mate successfully and their offspring can also mate successfully, all are the same species.