If a woman has blood type AB and her husband has blood type O, what is the percent chance that a child of theirs would have blood type A? (Hint: punnett square would be useful)

a. 25%
b. 50%
c. 66%
d. 75%

I cannot for the life of me remember anything about punnett squares and the information I have found thus far is not making things any clearer. Any help would be much appreciated. This is for my anthropology class. Thank you.