Let us simplify the argument given --
Premise1. the ability to transplant such organs as hearts, lungs, livers, and kidneys will allow us to live longer.
2. Cannot achieve brain transplant however
ConclusionThose with degenerative brain disorders will form an ever-increasing proportion of the population
This implies that even though we can achieve successful transplants of other organs, we cannot do so for the brain. This means that as people live longer (because of transplants, they will stop dying of other diseases), incidence of brain diseases would be higher -- since we cannot transplant brains.
Option A - Incorrectthe argument does not imply more and more young people will get brain disorders but rather that incidence of such disorders would be higher among people who live long.
Option B - IncorrectWe are not concerned about what is happening today, but the implications of what happens in the future when people start to live longer because of transplants.
By stating that people do not live long enough to receive more than one transplant, this sentence kind of
goes against the force of the argument. The argument clearly states that people will live longer as transplants become more common.
Option C - Correct answerIf degenerative brain disorders can be cured without transplants, the incidence of such disorders in the population as the people live longer will no longer be true.
Option D - IncorrectWe are not concerned about the percentage of death such disorders account for. But we are concerned with whether such disorders would be MORE common or not.
Option E - IncorrectWe cannot infer this nor is it relevant to the conclusion.