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

Answer: Option A

Donors with blood group O, Rh^- can donate to recipients with blood types A, B, AB and O (O is the universal donor), because it does not have any antigen, thereby no reaction would occur with the antibody, which may be present in donor's body. Hence, option A is correct.