When a man with blood group "AB" marries a woman with "0" blood group then blood group of children will be A' orB' which is differ from parental blood group. 

The RBC membrane contains either antigen A or antigen B, or both Antigen A and B or no antigen at all, accordingly the blood is classified as blood group A , B , AB and O respectively. The blood plasma of blood group A will have antibodies for B, that of B will have antibodies for A,  that of AB will have non, while that of O will have both the antibodies. If blood is transfused at random this will lead to antigen-antibody reaction - like if A is the blood group of the recipient while the blood group of the donor happens to be B, this will lead to clumping of the RBCs.

The cell membrane of RBCs in AB blood group contain both antigen A as well as antigen B therefore the blood plasma does not contain antibody A and Antibody B, to prevent the coagulation of blood cells due to antigen-antibody reactions.

If the boy has blood group o, both his parents could probably be heterozygous A group, $I^AI^O$, in both the parents.

Rh factor is an independent antigen present on the RBC irrespective of the presence of Antigen A or B. This factor is passed down from genus Rhesus macaque  (monkey) to humans and is regarded as the Rh factor it was discovered by Landsteiner and Weiner.

Type O ( specifically O negative ) blood is called the universal donor type because it is compatible with any blood type. O blood group does not contain any antigen hence does not result in transfusion reactions.

Rh incompatibility usually isn't a problem if it's the mother's first pregnancy because, unless there's some sort of abnormality, the fetus's blood does not normally enter the mother's circulatory system during the course of the pregnancy. However, during second child, the mother's and baby's blood can intermingle. If this happens, the mother's body recognizes the Rh protein as a foreign substance and might begin making antibodies (protein molecules in the immune system that recognize, and later work to destroy foreign substances) against the Rh proteins. Rh antibodies are harmless until the mother's second or later pregnancies. If she is ever carrying another Rh-positive child, her Rh antibodies will recognize the Rh proteins on the surface of the baby's blood cells as foreign, and pass into the baby's bloodstream and attack those cells. This can lead to swelling and rupture of the baby's RBCs. A baby's blood count can get dangerously low when this condition, known as a haemolytic or Rh disease of the newborn, happens.

If a woman who is Rh- negative married to a man who is Rh- positive produces a baby may have Rh- positive blood, inherited from the father. Rh antibodies are harmless until the mother's second or later pregnancies. If she is ever carrying another Rh- positive child, her Rh antibodies will recognize the Rh proteins on the surface of the baby's blood cells as foreign and attack those cells. This can lead to rupture of the baby's RBCs known as haemolytic or Rh disease of the newborn or erythroblastosis foetalis. 

When a woman is pregnant having Rh negative blood group and the blood group of the foetus is Rh positive, then her baby's blood type will be incompatible with her own. This can cause a condition known as erythroblastosis fetalis, where the mother's red blood cells attack those of the baby as there would any foreign invaders. Rh incompatibility and ABO incompatibility are the two main cause. Both are associated with blood type. Thus, the correct answer is option C.

Rh incompatibility arises when mother and the baby carries different protein factors. The problem arises when mother is Rh negative and the child she is carrying is Rh positive. This can take place even when two blood groups with different proteins are either mixed in a test tube or transfused into a person. Rh negative people do not have this protein and if Rh positive blood is mixed with Rh negative blood anti Rh antibodies develop which can lead to problems. 

Erythroblastosis fetalis also called the hemolytic disease of the newborn, type of anemia in which the red blood cells (erythrocytes) of a fetus are destroyed in a maternal immune reaction resulting from a blood group incompatibility between the fetus and its mother. Symptoms of erythroblastosis fetalis range from mild to severe; death of the fetus or newborn sometimes results.

The Rh system is responsible for the most severe form of the disease, which can occur when a Rh-negative woman (a woman whose blood cells lack the Rh factor) conceives a Rh-positive fetus. Sensitization of the mother’s immune system (immunization) occurs when fetal red blood cells that carry the Rh factor (an antigen in this context) cross the placental barrier and enter the mother’s bloodstream. They stimulate the production of antibodies, some of which pass across the placenta into fetal circulation and lyse, or break apart, the red blood cells of the fetus (hemolysis). Hence option B is correct.

There are two main causes of erythroblastosis fetalis: Rh incompatibility and ABO incompatibility. Rh incompatibility occurs when a Rh-negative mother is impregnated by a Rh-positive father. The result can be a Rh-positive baby. In such a case, your baby’s Rh antigens will be perceived as foreign invaders, the way viruses or bacteria are perceived. Your blood cells attack the baby’s as a protective mechanism that can end up harming the child. If you’re pregnant with your first baby, Rh incompatibility isn’t as much of a concern. However, when the Rh-positive child is born, your body will create antibodies against the Rh factor. These antibodies will attack the blood cells if you ever become pregnant with another Rh-positive baby.

Erythroblastosis foetalis is a disease of the new born which is caused by Rh incompatibilty. This happens when mother is Rh negative and the child she is bearing is Rh positive. Thus the Rh antigen of child can trigger the antibodies inside mother which can travel through circulation thus destroying foetus blood cells. Thus, correct answer is option D.