This may be tempting, because it does seem to jump to an extreme conclusion about the benefits of a certain course of action — switching entirely to a high-fiber diet will be healthier than a low-fiber diet. But it's important to see that the flaw here is different. This argument compares two options, a low-fiber diet and a high-fiber diet, and concludes that because one is healthier than the other, taking that diet to the extreme will be even more healthy.

In the stimulus, we didn't have a comparison of two different courses of action. There was just a statement of a positive trend, which was then used to claim that a certain process was entirely beneficial — i.e., had no negative effects — in a given context. So though this answer choice's conclusion may sound strong, it's actually not as extreme as the conclusion in the stimulus. Saying that an entirely low-fiber diet is "even more healthful" doesn't mean it has no negative benefits or is entirely beneficial. So this argument differs from the stimulus in structure and in the force of its conclusion.

This is what we're looking for. Based on a single statement about the benefits of exercise, the argument concludes that "no harm" can come from exercise: exercise is purely beneficial. Like the stimulus, this argument mentions a benefit of a course of action, and then assumes that there are no negative effects of that action that would outweigh its benefits.

Wrong flaw. This answer choice starts by mentioning a negative effect of a certain course of action, and then suggests one specific remedy for that effect (periodic fasting) to avoid that effect. The flaw here is that we don't know if the specific remedy actually helps avoid the negative effect of the first course of action — "fasting periodically" doesn't necessarily mean consuming less calories than one expends. Though definitely flawed, this is a different structure than our stimulus, which starts with a statement about one benefit of a course of action, and then claims that that course of action is purely beneficial.

Wrong flaw. This compares two different courses of action and assumes that one ought to commit entirely to one, at the total exclusion of the other. The stimulus only discusses one course of action, and is flawed because it states only one benefit of that course of action, then concludes that it yields entirely beneficial effects.

Though this might seem similar to the stimulus, in that it reaches an "absolute" or "extreme" conclusion (one should avoid penicillin altogether) based on a statement about one effect of a course of action, the structure here is very different. Remember that the stimulus mentioned a benefit of a certain course of action, and then ruled out any negative effects from that action, concluding that it was purely beneficial.

This answer choice starts with a negative effect of a course of action (taking too much penicillin) and then suggests a completely opposite course of action as a way of avoiding that effect and gaining a certain benefit, which the premises give no support for (strengthening the body's innate resistance). Though flawed, this is a very different structure from the stimulus.