[raymondh]

I think Feynman would be pleased with question. He was well grounded in the real world of doing science with incomplete datasets. For researchers who gather data with surveys, this is a frequently encountered problem, so it is legitimate to discuss it an statistics course.

If there is a problem, it is the multiple choice format. This issue of handling incomplete datasets would be make a good essay question.

That said, it is perfectly reasonable to choose D and work with the information you have. That is what real world clinic studies do when they report the number of people who dropped out of a study.

It is also reasonable to choose A and attempt to get better coverage. That is what real world election pollsters do.