Correct Option: C  [ the RBCs agglutinate ]
Remark:
Explanation: In an incompatible blood transfusion, the antibodies in the recipient’s plasma bind to the antigens on the donor’s RBCs. This causes the red blood cells to agglutinate (clump together) and eventually undergo hemolysis (bursting). The released hemoglobin can block kidney tubules and cause kidney failure, which can be fatal if untreated. This is why blood group compatibility testing (cross-matching) is essential before transfusions.
Example:
If a person with blood group B (having anti-A antibodies) receives blood from a person with group A (which has A antigens), the recipient’s anti-A antibodies will attack the donor’s RBCs, leading to agglutination.