The correct answer choice is (E)

The fact set in this stimulus is fairly compact, and includes the following two premises:

- 1. All John's friends say they know someone who has smoked 40 cigarettes a day for the past 40 years but is fit and well, and;

2. John doesn't know anyone like that and is sure that at least one of his friends doesn't know anyone like that either.

The question stem is a Must Be True, and thus we know the statements above are 100% true. If this is the case, we initially have a conflict because all of John's claim to know a heavy smoker who is quite fit but John says he doesn't, and that he's not unique among his friends. So, someone isn't telling the truth here. However, we actually can determine that it is john who is telling the truth because the stimulus states that "it is quite certain" that John is not unique. So, the stimulus notes that his friends "say" they know someone, but that John definitely does not know anyone like that and it is a fact that he is not alone in that. So at least one of John's friends is not telling the truth. Answer choice (E) states this directly and is correct, but let's review each answer to see why they are incorrect.

Answer choice (A): Smokers may often lie about how much they smoke, but it doesn't matter unless we know that the smokers John's friends know are lying about how much they smoke. John's friends could know only honest smokers that are telling them the truth. So this answer choice is incorrect.

Answer choice (B): Similar to answer choice (A), this is a general statement that doesn't affect or follow from John's friends or the supposed smokers that they know. The statements made by his friends, and the factual circumstances of what we know about John (as explained below in more detail in answer choice (E)) make it so that it is irrelevant whether people knowingly exaggerate. And in any event, we cannot know for sure that what occurred in the stimulus involves "knowingly exaggerat[ing] without intending to lie" —there's no proof of that occurring.

Answer choice (C): This answer choice could be true, but it does not have to be true. Since this is a Must Be True question, this answer is then incorrect.

Answer choice (D): This answer choice goes too far in saying "most." While we know at least one of his friends isn't telling the truth, we don't know that "most" of them aren't telling the truth, and thus this answer choice is incorrect. Another reason this answer cannot be correct is that if you select it as true, you automatically also force answer choice (E) to be true (since "most" automatically implies "some). Since you cannot have two correct answers to a problem, this answer is incorrect on that basis alone.

Answer choice (E): In LSAT terms, "some" means "at least one" or more, or anywhere between 1-100%. According to the stimulus, John is not alone in not knowing a smoker who fits the description in the stimulus, and so at least one of his friends (and thus "some" of his friends), must not be telling the truth.

Note that it can be difficult to pick answer choice (E) because it feels so powerful and definitive. It feels very strong (and it is), but that's acceptable because the stimulus has been very clear that John's situation is factual. Since his friends statements counter his, it has to be that at least one of them is not telling the truth.

If the "not telling the truth" aspect bothers you here, think about it this way: each of his friends was asked if they know a heavy smoker who was still fit. They each said yes. However, we know that John knows no one like this, and we also know that he's not the only one of his group who doesn't know a heavy smoker who was still fit. Thus, someone who said they know this smoker actually doesn't. Therefore, in saying they did, they lied.