The ABO blood system classifies human blood into four types based on the presence or absence of certain antigens on the surface of red blood cells. These types are A, B, AB, and O. For a safe blood transfusion, it's crucial that the donor's blood type is compatible with the recipient's. Compatibility is determined by antigens on the red blood cells and antibodies in the plasma. Here's how the compatibility works:

• Type A: Has A antigens on the red blood cells and anti-B antibodies in the plasma.
• Type B: Has B antigens on the red blood cells and anti-A antibodies in the plasma.
• Type AB: Has both A and B antigens on the red blood cells but does not have anti-A or anti-B antibodies in the plasma. People with AB blood are universal recipients for ABO transfusions because they do not have anti-A or anti-B antibodies.
• Type O: Has no antigens on the red blood cells but has both anti-A and anti-B antibodies in the plasma. People with O blood are universal donors for ABO transfusions because they have no A or B antigens on their red blood cells.

Given the scenario where the recipient's blood type is unknown, the safest option for blood transfusion would be Type O blood. This is because Type O blood has no A or B antigens on the red blood cells, making it compatible with any recipient regardless of their ABO blood type. Thus, regardless of the recipient's unknown ABO blood group, a friend with Type O blood could donate without causing a harmful reaction due to ABO incompatibility.

Therefore, the correct answer is:

Option C: Type O