The answer is c. alleles that are carried on X and Y chromosomes.
Sex-linked traits are those traits which inheritance depends on sex chromosomes. Sex chromosomes are X and Y. Women have two X chromosomes, while men have one X and one Y chromosome. One those chromosomes are genes responsible for some sex-linked traits. As genes on the other chromosomes, genes on sex chromosomes as well have different forms called alleles. Therefore, alleles that are carried on X and Y chromosomes (sex chromosomes) best describe sex-linked traits.
