The correct choice is (a) Antigen and antibody
To explain: The blood groups have antigens which produce antibodies for the opposite blood group. Thus, blood group A has antigen A and it produces antibody B against the blood group B which contains antigen B. When a blood test is done, the blood of group A will clump up when antigen B is added to the taken blood. In the same way, the blood of group B will clump when antigen A is added. For blood group AB, no clumping happens and for group O, clumping happens for both Antigen A and B.