The correct option is B Rh factor
Erythroblastosis fetalis is also known as erythroblastosis neonatorum. It is hemolytic anemia in the fetus. It is caused when a Rh positive mother has the Rh-negative baby. Baby’s Rh antigens will be perceived as foreign and mother's body produces antibody against the Rh negative antigen and attacks the blood cells of the baby which results in anaemia. It is also caused due to incompatible blood group. When a mother with an O blood type gives birth to a baby who has an A, B, or AB blood type. Because O blood types have both A and B antibodies, the mother’s blood can attack the baby’s blood cells.