The correct answer is A. O, Rh-.
Blood is classified into four main groups: A, B, AB, and O. Each blood group is further classified as Rh- or Rh+. The Rh factor is a protein that can be found on the surface of red blood cells. People who have the Rh factor are Rh+, while people who do not have the Rh factor are Rh-.
In general, people with blood type O can donate blood to people with any blood type. This is because blood type O does not have any antigens (proteins that the immune system can attack). People with blood type AB can only receive blood from people with blood type AB. This is because blood type AB has both A and B antigens. People with blood type A or B can only receive blood from people with the same blood type.
In the case of an emergency, when the blood type of the patient is not known, it is safest to give blood type O, Rh-. This is because blood type O is the most common blood type and is compatible with all blood types.
It is important to note that blood transfusion is a medical procedure that carries risks. It is important to discuss the risks and benefits of blood transfusion with your doctor before making a decision.