In humans multiple alleles are involved in the inheritance of?

The best answer is probably blood types, A, but the question is worded poorly. "Multiple" means more than 1, and technically both answers A and D involve "multiple" alleles. Standard blood types you learn in high school involve several alleles from two loci. The text book example for sickle cell involves two alleles for one locus. The question was likely meant to read "multiple loci," which is why I think A is the best answer.