Final answer:
The Landsteiner law states that individuals have antibodies in their serum against the antigens they lack.
Step-by-step explanation:
The correct answer is a. Landsteiner.
According to Karl Landsteiner's discovery of the ABO blood group system, individuals have antibodies in their serum against the antigens they lack.
This means that if someone lacks antigen A, they will have anti-A antibodies, and if someone lacks antigen B, they will have anti-B antibodies in their blood plasma.