afung22

I think D. The explanation is a little bit different though:

ABO gene expresses either A allele or B allele (whose protein product converts H antigen to either A or B antigens -more info on wikipedia-). O allele is created by the mutation that you talked about. One bp deletion causes frame shift in the expression of this protein which is not functional for processing blood type antigens.

you can think of this as a defective gene which has no function.

when there is another deletion in the same codon (second bp in the triplet) it will still cause a frameshift (even though a different protein will be translated due to the shifted reading frame). You can think of this as another defective gene.

the person will end up inhering two O alleles. Blood type will be O; BUT NOT because there is a "functional copy of O allele. Instead the person will have two defective copies which are unable to process blood type antigens.

hope that helps.