Wrong flaw. (A) initially gives us two separate instances where it’s okay to dress casually, which does parallel the stimulus. Instead of concluding that it’s okay to dress casually when these two instances are combined, however, it concludes that when these two instances are not present, it’s likely not okay to dress casually. The stimulus tries to prove that it is sometimes okay to do two things together because it is sometimes okay to do them separately, so (A) is the opposite of what we’re looking for.

Wrong flaw. (B) sets up a conditional relationship: if five people help you, and you don’t need to take it up or down stairs, then it’s usually easy to move a piano. It then concludes the presence of the necessary condition without establishing the presence of either part of the sufficient condition. The stimulus, meanwhile, concludes that it’s sometimes okay to do two things together since it’s sometimes okay to do them separately, so (B) doesn’t match.

(C) concludes that it’s okay to do two things together (take drugs A and B for headaches) because it’s okay to do them each separately. The stimulus concludes that it’s sometimes okay to do two things together since it’s sometimes okay to do them separately, so (C) matches.

Wrong flaw. (D) tells us that the ability to safely drive a truck can be impaired by certain prescription drugs. This does not support the conclusion that it’s never safe to drive a truck while taking prescription drugs. The stimulus, meanwhile, concludes that it’s sometimes okay to do two things together since it’s sometimes okay to do them separately, so (D) doesn’t match.

Wrong flaw. (E) sets up a conditional relationship: if it rains, the mountain roads become treacherous and the streams become full. It then invalidly concludes that the presence of these necessary conditions (treacherous roads and full streams) indicates the presence of the sufficient condition (rain). The stimulus, meanwhile, concludes that it’s sometimes okay to do two things together since it’s sometimes okay to do them separately, so (E) doesn’t match.