Answer choice E

This is indeed a tough one, and it took me a minute to figure out why E is necessary. Let's try to negate it by saying that taking unsafe amounts do give an advantage. The argument is that there will always be an incentive to cheat, so why not let people cheat but in a safer way. The negation of E shows that the incentive to cheat further will still be present despite allowing the use of PEDs safely, and that the cheating will remain unsafe. This doesn't exactly destroy the conclusion, but it does undermine the reasoning for the argument. The argument is that people will cheat, so let's make it safe. If we negate E then we know that allowing peds wont actually make it safe.

I think I may be just too inflexible in my approach. When only one negation undermines the conclusion and none collapse it, then I should select the one that undermines.

Fair enough although I will say this is a bit atypical of a necessary assumption question. The right answer doesn't collapse the conclusion but it does undermine the rationale for making the argument as a whole, and I can only remember one or two others like this. Most of the time it will indeed collapse the argument