Final answer:
Peptide binding diversity is increased by both multiple MHC alleles in a population and multiple homologous MHC genes in the genome, which help in recognizing a wide variety of antigens.
Step-by-step explanation:
The peptide binding diversity in the context of the immune system is increased by multiple MHC alleles in the population and multiple homologous MHC genes in the genome. This diversity is essential for the recognition of a wide variety of antigens. The presence of multiple alleles and genes creates a larger repertoire of possible peptide-MHC combinations, thus enhancing the immune system's ability to recognize and respond to a broad range of pathogens. It's not influenced by the size of the peptides, as these typically bind within a groove of the MHC molecule that accommodates peptides of a certain length range. Hence, the correct answer to the question is D. A and B.