A man whose blood group is not known meets with a serious accident and needs blood transfusion immediately. Which one of the blood groups mentioned below and readily available in the hospital will be safe for transfusion?

1. The question involves an emergency blood transfusion for a person with an unknown blood group. 2. In emergencies where the patient's blood type is unknown, the safest option is to transfuse 'universal donor' blood. 3. The universal donor blood group in the ABO system is Type O, as these red blood cells lack A and B antigens and are less likely to provoke an immune reaction in recipients of other blood types (A, B, AB). 4. The Rhesus (Rh) factor is also critical. Rh-negative blood lacks the RhD antigen. Transfusing Rh-negative blood is safer for recipients whose Rh status is unknown, as it avoids sensitization in Rh-negative recipients and is compatible with Rh-positive recipients. 5. Therefore, blood group O, Rh-negative (O-) is considered the universal donor type and is the safest for transfusion in emergencies when the patient's blood group is unknown. 6. O, Rh+ is not universally safe because the Rh+ factor can cause a reaction in Rh- recipients. AB blood types (Rh- or Rh+) are universal recipients (can receive from any type) but not universal donors.