The frequency of X-linked recessive trait in a population with XY sex-determination system is 0.3. What will be the frequency of affected males?

The correct option is B 0.3
In the XY system of sex determination, females are homogametic (XX) and males are heterogametic (XY). Since the males have only one X-chromosome the frequency of X-linked alleles in the population is equal to the frequency of affected males.
As the frequency of a recessive allele is 0.3 so the frequency of affected males is 0.3 in that population.