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

 - normal wings
 - vestigial wings
The fruit fly's genotype for the wing type gene is nn. The fruit fly's genotype of nn has only n alleles. The n allele is for vestigial wings. So, the fruit fly's phenotype for the wing type trait must be vestigial wings.
To check this answer, consider whether the fruit fly's alleles are dominant or recessive. The allele for normal wings (N) is dominant over the allele for vestigial wings (n). This means N is a dominant allele, and n is a recessive allele.
The fruit fly's genotype of nn 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 wing type trait must be vestigial wings.