Which Mother-Fetus Combination Is at Most Risk for a Maternal-Fetal

Which mother-fetus combination is at most risk for a maternal-fetal blood incompatibility?

A) Mother is Rh-positive, and fetus is Rh-negative.
B) Mother is Rh-negative, and fetus is Rh-positive.
C) Mother has type A blood, and fetus has type O blood.
D) Mother has type AB blood, and fetus has type B blood.