Which blood group is called "Universal donor"?

The correct option is D O
Blood group O is known as the universal donor as this blood group does not contain any antigens on the surface of its red blood cells. It lacks both A-antigen as well as B-antigen. Thus the individuals with blood group O can transfuse their blood to people of all blood groups as no antibodies will get formed in the recipient person's body. Antigens are the proteins which trigger the formation of antibodies in the person's body causing sticking of red blood cells. The persons with blood groups A, B and AB all have A-antigen, B-antigen and both A and B antigen respectively which can cause an allergic response in the body.

The person with blood group AB is considered as the universal recipient as the blood of these people contains no antibodies even if any of the A-antigen or B-antigen enters their body. Both the antigens are considered as self for these people. Thus they can take blood from any person which would not cause any allergic response in the body as they do not contain any of the antibodies.

Thus, the correct answer is option D.