In the ABO blood group system, there are two main antigens (proteins on the surface of red blood cells) that are considered: antigen A and antigen B. An individual's blood type in this system is determined by the presence or absence of these antigens. Additionally, in the bloodstream, there will usually be antibodies against the antigens that the individual does not express. For example, someone with antigen A will typically have anti-B antibodies, and someone with antigen B will typically have anti-A antibodies. A person with neither antigen will have both types of antibodies, and a person with both antigens will have neither type of antibodies.

If both antigens are present (antigen A and antigen B) on the surface of the red blood cells but no antibodies against them are found in the plasma, this signifies that the blood group of the individual is AB. This is because the presence of both antigens means the immune system does not need to produce antibodies against them since the body recognizes both antigens as self.

Therefore, the correct answer is:

Option C: AB