The correct answer is option 3: O. The blood group O is considered the universal donor because individuals with this blood group can donate blood to individuals of any other blood group (A, B, AB, or O) without the risk of a transfusion reaction. This is because type O blood does not contain A or B antigens on the surface of red blood cells.
Option 1: A blood group has A antigens on the surface of red blood cells, so it is not a universal donor.
Option 2: B blood group has B antigens on the surface of red blood cells, so it is not a universal donor.
Option 4: AB blood group has both A and B antigens on the surface of red blood cells, so it is not a universal donor either.
Hence, the correct answer is option 3: O.