Which combination of alleles best describes a sex-linked trait?
a. heterozygous alleles
b. homozygous alleles
c. alleles that are carried on x and y chromosomes
d. alleles that are carried on somatic cells

B. homozygous alleles 

Each human body is composed of genes, and these usually come in pairs that are slightly different from each other. The different versions of genes are called alleles. Alleles are responsible for determining physical or notable characteristics and traits a specific person has. 

The answer would be c, white eyes are homozygous recessive. in the early 1900s, morgan conducted several experiments with vinegar  flies (drosophila). one  sample revealed an anomaly: a  white-eyed, single male fly. he attempted to breed this fly with the typical red-eyed females. the offspring were all red-eyed. using flies from this new generation (brother-sister pairs)  produced some red-eyed and a few white-eyed flies. the white-eyed were all male. in later experiments, he was able to produce white-eyed females. this proved that white eyes are not lethal to either sex (ruling out a and b). since the percentage of white eyes to red eyes is so low, white eyes are not a dominant trait. it would be a recessive trait.