What is one possible genotype for a patient who develops anti-C antibody?

When a patient develops an anti-C antibody, it indicates that the individual has the C antigen on their red blood cells and has been sensitized to the antigen, leading to the formation of the antibody against it.

The C antigen is part of the Rh blood group system. In order to demonstrate the anti-C antibody, the patient must lack the C antigen while possessing the corresponding gene that encodes for it. Therefore, individuals with the genotype "rr" do not express the C antigen, as they carry the "r" allele that does not code for the C antigen.

On the other hand, individuals with other genotypes such as R1R1, R1r, and r'r can express the C antigen on their red blood cells due to the presence of at least one "R" allele associated with the C antigen. This presence would typically prevent the development of anti-C antibodies unless they have been exposed to the C antigen through transfusion or pregnancy.

Thus, "rr" is the correct answer, as it represents a genotype where the patient does not express the C antigen and is therefore capable of producing anti-C antibodies upon exposure to it.