I think it is 24 because it just is
If the parents have black hair then all the kids would have black hair (a)
if one parent has blonde hair and the other has black hair then 2 kids would be blonde and 2 would have black hair (d)
if the parents have blonde hair then the kids would have blonde hair (c)
Chromatids thats your answer to that question.
Yes, this is true, and it is actually one of the organisms driving evolution! Some organisms develop mutations that are actually beneficial (better eye-sight, for example) and they're more likely to pass those to their children that individuals without those mutations!