Explanation
The herbalist posits an argument regarding herbal medicines and toxic side effects. namely the relationship between the side effects and the frequency and duration of use. We are asked to find an assumption required by the argument.
A. (Correct Response) For the herbalist's recommendation to skip doses to be relevant, it must be assumed that there are indeed people who use herbal medicines daily for long enough periods that toxic side effects could manifest. If no one used them for such durations, the conclusion would not be hold because this advice would not apply to anyone. At least some people is the key phrase in this stimulus.
B. The argument doesn't hinge on the effectiveness of the medicines being reduced by skipping doses, but rather on the avoidance of side effects.
C. The assumption that some herbal medicines have toxic side effects even if doses are skipped is not a part of the herbalist's argument, which suggests taking breaks can be beneficial.
D. While the herbalist does suggest that herbal medicines should be given a chance to work over several months, this is not presented as an assumption but rather as part of the premise.
E. The assumption is not about the conditions under which one should skip doses, but rather about the existence of people who are at risk of side effects due to their usage patterns.
Option A is necessary for the conclusion to hold; otherwise, the recommendation to occasionally skip doses would not apply to anyone.