Irrelevant. Remember that on the LSAT, we can take the statements in premises as established, unless the stimulus itself gives us some reason to doubt them. So when the author states that it is "undeniable" that excessive candy eating causes tooth decay, we can take that as a given, without demanding explicit references to science.

Incorrect. The author never claims that everyone with cavities must have gotten them from eating too much candy. His focus in the analogy is on "candy eaters with cavities," not everyone with cavities.

It's true that these terms could be defined more specifically, but this argument doesn't require that. If the author said "tobacco companies should be held liable for health problems caused by a certain period or threshold of long-term smoking, just like candy companies should be held liable for tooth decay caused by a certain threshold amount of excessive candy eating", then the specific meaning of each term would be more relevant.

As it stands, though, the ideas of "excessive" and "long-term" aren't actually what is being compared in the analogy. What's more relevant is the general pattern of both behaviors potentially causing health problems.

Irrelevant. The author doesn't need to state the identity of these people to interact with their argument.

This is correct, though it might seem unexpected. This answer choice focuses on a difference that effectively undermines the author's analogy, but might not be as obvious on an initial read of the stimulus: specifically, the different magnitude of the harm caused by eating candy versus smoking, and how this might affect what people believe about legitimate actions based on such harm.

Just because "no one seriously believes" you should be able to sue candy companies for cavities, it doesn't follow that one cannot seriously believe a tobacco company should be sued for cancer. If candy could also be linked to cancer instead of a comparatively less harmful problem like tooth decay, perhaps people would think it was legitimate to sue candy companies. Thus, the different scale of the health problems undermines the analogy the author uses as support.

In terms of strategy, even if you didn't pre-phrase this particular answer choice, the fact that it does attempt to point out a difference between the things being compared in the stimulus should make you slow down and ask whether this difference is actually relevant to the stimulus. In this case, and unlike A (where the difference isn't relevant), this answer choice successfully points out a flaw in the analogy.