The correct answer is option 2, O-. O- is considered the universal donor blood type because it lacks the A, B, and Rh antigens on the surface of red blood cells. This means that O- blood can be safely transfused into individuals with any blood type, as it is unlikely to cause an immune response. On the other hand, individuals with blood types A, B, or AB have either A or B antigens on their red blood cells, and individuals with the Rh factor have the Rh antigen. Therefore, these blood types can only receive blood from donors who do not have the antigens that their own blood type lacks. It is important to note that even though O+ blood is a common blood type and can be given to many people, it is not considered a universal donor type because it has the Rh antigen present.