Erythroblastosis foetalis can occur if:

The correct option is A Mother is Rh-ve and foetus is Rh+ve
Erythroblastosis foetalis caused by transplacental transmission of maternal antibodies to fetal red blood cells. The disorder usually results from incompatibility between maternal and fetal blood groups. It generally occurs when mother's blood is Rh–ve. Suppose in her first pregnancy, the first child (foetus) is Rh+ve. In this case, the child (foetus) does not suffer because antibodies against Rh+ve are not produced. When baby's blood mixes with mother's blood during pregnancy, immune system of the mother reacts by making antibodies against Rh+ve. But in the subsequent Rh+ve foetuses, the antibodies against Rh antigen of the mother blood destroy the fetal RBC. This results in erythroblastosis foetalis.