Explanation
The blood group that is known as the universal donor is blood group O. This is because blood group O does not have any antigens on its surface, which means that it can be transfused into individuals with any other blood group without causing an immune reaction. Therefore, people with blood group O are often referred to as "universal donors."
In contrast, blood group AB is known as the universal recipient because it can receive blood from individuals with any other blood group without causing an immune reaction. Blood group A and B have antigens on their surface, so they can only receive blood from individuals with the same or compatible blood groups.
Therefore, the answer is A: O.
