Based on this information, what is Mabel's phenotype for the leg color trait?

 - white legs
 - yellow legs
Mabel's genotype for the leg color gene is ll. Mabel's genotype of ll has only l alleles. The l allele is for yellow legs. So, Mabel's phenotype for the leg color trait must be yellow legs.
To check this answer, consider whether Mabel's alleles are dominant or recessive. The allele for white legs (L) is dominant over the allele for yellow legs (l). This means L is a dominant allele, and l is a recessive allele.
Mabel's genotype of ll has only recessive alleles. An organism with only recessive alleles for a gene will have the recessive allele's version of the trait. So, Mabel's phenotype for the leg color trait must be yellow legs.