Pedigree question bio

answer is C
I know it should be a smaller ratio then 25 percent because we don’t know wether the female is a carrier or not for the second generation. but can someone just give me the equation to solve it


the mother (II 2) has a 1/2 chance of being a carrier, she can only be °XX or XX
the offspring has 1/4 chance of being color blind °XX XX XY °XY
1/2 * 1/4 = 1/8 = 12,5%