I'm not sure but I would go with C. if I were you because s' is possession of a group of people for example All the moms' children in the school were punished moms' is possession of more than one mom but mom's is possession of one mom so if you say childrens' instead of children's that would be wrong because children is a plural by itself so you can't turn a plural into a plural possession all you have to do is just put 's after a plural word to give it possession.
