Answer:
The correct answer is b. Rh factor incompatibility.
Step-by-step explanation:
Rh factor incompatibility occurs when a mother's body makes antibodies against the Rh antigen present in fetus blood which can lead to fetal death if not treated.
Commonly it occurs when an Rh⁻ woman carries a fetus which is Rh⁺. During first pregnancy complications are not serious because memory was not built against Rh⁺ antigen and only during the delivery mother's blood comes in contact with fetal blood.
But during second pregnancy memory was already present in the mother's body against Rh⁺ antigen because of the earlier exposure during first pregnancy, therefore, a quick and enhanced immune response is generated against the fetal blood antigen which results in erythroblastosis fetalis.
Therefore the right answer is b. Rh factor incompatibility.