This is not my forte, and i could use some insight

q: colourblindness is a sex-linked recessive trait. if a colour blind man and woman that is a carrier for the trait have two girsl and two boys, what is the probability that at least one of the girls will be colourblind?

a) 0%

b) 50%

c)75%

d)100%

The answer is c)...how does one come up with that?? Please and thank you!