The correct answer is option 4: O. A universal donor is someone with blood group O. This means that their blood can be given to people with any other blood types (A, B, AB, or O) without causing adverse reactions. The reason for this is that the blood of individuals with group O does not have A or B antigens on the surface of the red blood cells. Since the presence of these antigens determines the compatibility of blood transfusions, individuals with blood group O can donate to anyone without their antibodies attacking the recipient`s blood cells. In contrast, individuals with blood types A, B, or AB have antigens on their blood cells that may be recognized as foreign by the immune system of the recipient, leading to potential complications.