Probable cause of Erythroblastosis fetalis can be

The correct option is Adjoining of RBC.

Erythroblastosis fetalis, also known as hemolytic disease of the newborn, is an alloimmune condition that occurs when there is an incompatibility between the blood types of the mother and the fetus, specifically involving the Rh factor (Rh-negative mother and Rh-positive fetus).

During pregnancy or delivery, fetal red blood cells (RBCs) carrying the Rh antigen can enter the mother's circulation. Since the mother is Rh-negative, her immune system recognizes these Rh-positive fetal RBCs as foreign and produces antibodies against them.

In subsequent pregnancies with an Rh-positive fetus, these maternal antibodies cross the placenta and bind specifically to the antigens on the fetal RBCs. This binding or adjoining of antibodies to the red blood cells leads to their agglutination (clumping together) and subsequent destruction (hemolysis). Therefore, the adjoining of antibodies to RBCs and the clumping/adjoining of RBCs themselves leads to their destruction, which is the primary cause of the clinical manifestations of erythroblastosis fetalis.