The correct answer is Option A: No antigens but both a and b antibodies are present.

Explanation:

In the ABO blood group system, blood group O is characterized by the absence of antigens on the surface of the red blood cells. Specifically, it does not have A or B antigens. However, the plasma of people with blood group O contains both anti-A and anti-B antibodies. These antibodies are ready to act against the A and B antigens if they are introduced into the body, such as via a blood transfusion with an incompatible blood type.

Thus, for blood group O:

• Antigens present on red blood cells: None (neither A nor B antigens)
• Antibodies present in the plasma: Both anti-A and anti-B antibodies

This unique feature allows individuals with blood group O to donate blood to any other group (universal donor), but they can only receive blood from other group O individuals due to the presence of both anti-A and anti-B antibodies in their plasma.