Males are more often affected by sex-linked traits than females because
a) male hormones such as testosterone often alter the effects of mutations on the x chromosome
b) female hormones such as estrogen often compensate for the effects of mutations on the x chromosome
c) x chromosomes in males generally have more mutations than x chromosomes in females
d) males are hemizygous for the x chromosome

D) males are hemizygous for the X chromosome

Explanation:

Both males and females have two sex chromosomes, but females have two X chromosomes and males have one X chromosome and one Y chromosome. When only one copy of a gene is present in the cell of a diploid individual, that gene is in hemizygous condition.

A female can be heterozygous for a sex-linked trait, one X chromosome carrying a dominant allele and the other X chromosome carrying the recessive allele. In that case, the expression of recessive allele will be masked by the expression of the dominant allele.Males, however, only have one X chromosome, so any allele it carries will be expressed, regardless of its dominance. For that reason, the frequency of appearance of sex-linked traits will be higher in males than in females.