Based on this information, what is Stacy's phenotype for the sickle-cell disease trait?

 - not having sickle-cell disease
 - having sickle-cell disease
Stacy's genotype for the sickle-cell disease gene is aa. Stacy's genotype of aa has only a alleles. The a allele is for having sickle-cell disease. So, Stacy's phenotype for the sickle-cell disease trait must be having sickle-cell disease.
To check this answer, consider whether Stacy's alleles are dominant or recessive. The allele for not having sickle-cell disease (A) is dominant over the allele for having sickle-cell disease (a). This means A is a dominant allele, and a is a recessive allele.
Stacy'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, Stacy's phenotype for the sickle-cell disease trait must be having sickle-cell disease.