The correct option is A Chemical reaction
The Rh system has been shown to be involved in transfusion reactions and also in affections such as the hemolytic disease of the newborn (HDN), autoimmune hemolytic anemias, and hemolytic reactions of nonimmune origin. HDN occurs when an Rh-negative mother carries an Rh-positive fetus. Although the fetal red blood cells are separated from the mother’s circulation by the placenta, during late pregnancy fetal red blood cells may escape into the mother’s circulation where they are perceived as foreign and provoke an antibody response. So, the correct option is "A" ( Chemical reaction).