The correct answer is O.
Blood type O is the universal donor because it does not have any A or B antigens on the surface of the red blood cells. This means that it can be safely donated to people with any blood type.
Blood type A has A antigens on the surface of the red blood cells. It can be donated to people with blood type A or AB.
Blood type B has B antigens on the surface of the red blood cells. It can be donated to people with blood type B or AB.
Blood type AB has both A and B antigens on the surface of the red blood cells. It can only be donated to people with blood type AB.
It is important to note that blood type is determined by the presence or absence of certain antigens on the surface of red blood cells. These antigens are proteins that the immune system can recognize. If a person receives blood that is incompatible with their blood type, their immune system will attack the donated blood, which can lead to serious health problems.
For this reason, it is important to know your blood type and to only receive blood from a donor with a compatible blood type.