You might be tempted to pick this answer choice if you thought that the last sentence implied that Professor Jameson prefers Aurora because she is an expert on the physiology of smell. In other words, if you read the last sentence as saying that experts on the physiology of smell must prefer Aurora to other perfumes (experts → prefer Aurora), then the contrapositive would indeed tell us that anyone who does not prefer Aurora to other perfumes, which would include Joan's friends, must not be an expert on the physiology of smell.

But that's not how the last sentence works. We only know that Professor Jameson is one of the world's experts on the physiology of smell, and happens to prefer Aurora to other perfumes.

But this leaves room for some experts to not prefer Aurora to other perfumes, which means it's possible for some of Joan's friends to be experts. If you picked this answer choice because the statement that Joan's friends "must have a defect in their sense of smell" made it seem unlikely that they were experts in the physiology of smell, that doesn't follow logically — you don't need a perfect sense of smell to be an expert in the physiology of smell — and besides, we're looking at the information presented to support the conclusion, not at the conclusion itself.

This doesn't follow. We only know that Aurora smells worse to Joan than any perfume in a comparable price range. In other words, all other perfumes comparable in price to Aurora smell better to Joan than Aurora. This doesn't mean that all perfumes smell better to Joan than Aurora. She might prefer Aurora to some perfumes that are much cheaper, for example.

This is a valid inference, and the phrasing of this answer choice helps us figure out where to start analyzing the stimulus in terms of conditional logic. Remember what we know about Joan's friends: none of them like Aurora as much as other perfumes. In other words, they all like Aurora less than other perfumes. By the contrapositive, someone who likes Aurora as much or more than other perfumes must not be one of Joan's friends:

JF → like Aurora less

/like Aurora less → /JF

Since Professor Jameson prefers Aurora to all other perfumes, we can therefore infer that she cannot be one of Joan's friends.

This doesn't follow. We're only told that none of Joan's friends likes Aurora as much as other perfumes. This doesn't mean they don't like Aurora at all.

This doesn't follow. We're told about Joan and her friends' dislike for one specific perfume, Aurora, compared to other perfumes. This doesn't tell us anything about the types of perfumes they actually like, or whether they like the same types of perfumes or not.