A limited supply of the mixture could explain why many people don’t use it even if it were effective. So to support the conclusion of ineffectiveness, the sufferer must assume that there’s enough of the mixture to supply most people.

Negative side effects could deter many people from using the mixture, so the sufferer must assume that this isn't the case. Otherwise, the mixture could absolutely be effective even though most people don't use it.

The sufferer claims that the mixture is not effective as a remedy for the common cold, but never discusses prevention. Because the argument is just about curing existing colds rather than preventing future ones, there’s no need to assume that the mixture prevents future colds; prevention is beyond the scope of the argument. And since this is an except question, (C) not being necessary makes it the correct answer.

A general lack of awareness could explain why many people don’t use the mixture, so the argument must assume that people know about it. Without this assumption, it would be impossible to conclude that the mixture is ineffective just because people don't use it.

If it’s possible that there are multiple effective remedies, and people simply chose other effective remedies over this mixture, then the mixture could still be effective. So to support the conclusion that the mixture is ineffective based on people's lack of use, it's necessary for the sufferer to assume that (D) is the case.