Correct Option (A)
In emergency situations where the patient's blood group is unknown, O, Rh- (O negative) blood is considered the safest option for transfusion. This is because O negative red blood cells lack A and B antigens on their surface, as well as the Rh factor antigen. The absence of these antigens minimizes the risk of an immune reaction in the recipient, regardless of their ABO or Rh blood type. Consequently, O negative blood is known as the universal donor.
Incorrect Options:
O, Rh+ (O positive) blood contains the Rh factor. If transfused to an Rh-negative recipient, it can cause Rh sensitization and a severe immune reaction, especially in subsequent transfusions.
AB blood, whether Rh+ or Rh-, contains both A and B antigens on its red blood cells. Transfusing AB blood to individuals with A, B, or O blood types would trigger a severe immune response due to the recipient's pre-existing anti-A and/or anti-B antibodies reacting with the donor antigens. AB blood is considered a universal recipient, meaning individuals with AB blood can receive blood from any ABO type, but it is not a universal donor.