A sex-linked disorder means that the gene associated with it is found in the X chromosomes, so females with the defective gene express the condition only when two defective copies are present, but males express the condition with just one copy of the defective gene.
The Punnett's square for this couple would be as follows:
If the couple has a MALE child, then the probability that the child has DMD is 50%, which means c, 50% is the right answer.