E is the best.
This question, although it's pretty bad, is not as bad as some other "150 difficult questions".
Premise: Tests are done to the about-to-be transplanted organs.
Conclusion: The possibility of rabies, contained in transplanted organs, affecting recipient is zero.
Proposed Assumptions - Things never go wrong either during the transplant process or during the testing.
A), This simply repeats the background of the question. It's irrelevant.
B), The number of people who are willing to donate organs has nothing to do the incident that people get affected by rabies after receiving donated organs.
C), We don't know when those people get affected by rabies. If this question is: which of the following is necessary to be done to evaluate the argument, this option could be an ideally correct answer.
D), This tells us how rabies are transmitted. It's irrelevant to the reasoning.
E), One more step needs to be done to understand this question. As I mention about the assumption - Things can never go wrong during the process of transplanting or testing. This option attacks "testing process". Simply, how will we know that people will always be able to differentiate the effects of cocaine and the effects of rabies through testing? Remember, the conclusion is that the possibility is "non-existent". One more step here is not to make another assumption to make the assumption in the argument work more properly, but rather to connect this option with the proposed assumption: There must be something between this similarity and the testing that will make such "non-existent" not so absolute.
Feel free to discuss.