Based on this information, what is this fly's phenotype for the antenna type trait?

 - mutated antennae
 - normal antennae
The fruit fly's genotype for the antenna type gene is aa. The fruit fly's genotype of aa has only a alleles. The a allele is for normal antennae. So, the fruit fly's phenotype for the antenna type trait must be normal antennae.
To check this answer, consider whether the fruit fly's alleles are dominant or recessive. The allele for normal antennae (a) is recessive to the allele for mutated antennae (A). This means A is a dominant allele, and a is a recessive allele.
The fruit fly's genotype of aa 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 antenna type trait must be normal antennae.