The scenario in Shaw’s quote describes physician’s who are paid for a surgical procedure, even if that procedure is extreme and unnecessary. The example here is the removal of a leg – a frightening prospect if a surgeon can get paid anytime he or she cuts off a leg.
To find a case that employs similar reasoning, we want an answer choice containing an occupation in which a person is paid for a procedure, even when that procedure is unwarranted. Basically, the person is motivated by pecuniary interest, i.e. money.
Answer Choice A clearly mirrors this – we have a dentist performing unnecessary procedures because he/she will profit from them.
As for Choice D, oil companies are not performing an unnecessary procedure just to make money. Their profits result from an economic strategy, i.e. lowering the price of oil.
Hope that helps!