Based on this information, what is this fly's phenotype for the body color trait?

 - a gray body
 - a black body
The fruit fly's genotype for the body color gene is bb. The fruit fly's genotype of bb has only b alleles. The b allele is for a black body. So, the fruit fly's phenotype for the body color trait must be a black body.
To check this answer, consider whether the fruit fly's alleles are dominant or recessive. The allele for a gray body (B) is dominant over the allele for a black body (b). This means B is a dominant allele, and b is a recessive allele.
The fruit fly's genotype of bb has only recessive alleles. An organism with only recessive alleles for a gene will have the recessive allele's version of the trait. So, the fruit fly's phenotype for the body color trait must be a black body.