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

 - a blue body
 - a greenish-brown body
The rainbow trout's genotype for the body color gene is bb. The rainbow trout's genotype of bb has only b alleles. The b allele is for a blue body. So, the rainbow trout's phenotype for the body color trait must be a blue body.
To check this answer, consider whether the rainbow trout's alleles are dominant or recessive. The allele for a greenish-brown body (B) is dominant over the allele for a blue body (b). This means B is a dominant allele, and b is a recessive allele.
The rainbow trout'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 rainbow trout's phenotype for the body color trait must be a blue body.