This is a Flaw question. Our job is simply to figure out what's wrong with the argument.
To start, we need to find the conclusion. Here, the conclusion is part of the last sentence starting with “most” and ending with “day.” We then want to see whether the premises adequately prove the conclusion (Spoiler: They don't. They never do in a flaw question.) For this particular question, the premises state that aspirin helps to thin the blood. Thinning the blood helps with heart disease and heart disease is one of the most common ailments in industrialized nations. However, the conclusion then jumps from talking about a single ailment (heart disease) to health in general. Additionally, the premises talk about the “most common” type of illness, whereas the conclusion says “most people.” There is no reason to think that most people have the most common type of illness or are even susceptible to this type of illness. (Perhaps illness is generally very rare, in which case "most people" might have no illness at all.)
A. The argument doesn't "take this for granted." The studies it references have "consistently found" that aspirin can prevent heart disease.
B. This is the correct answer, as predicted above. To illustrate this difference further, take China. China is the most populous nation, but that doesn't mean China has most of the world's population in it.
C. This is very close to a correct answer, but it is slightly off. The argument overlooks the possibility that aspirin would negatively impact individuals health in some other way, and that is a flaw. However, this answer choice just says that the argument overlooks the possibility that it would have no effect on other diseases, which is not a flaw. If Aspirin has no effect, then that doesn't make the argument problematic at all.
D. The argument does overlook this, but that doesn't matter. The conclusion doesn't say that taking an aspirin is the best way to increase health or prevent heart disease, just that it will do so.
E. The argument does fail to address this possibility, but that is not an issue. We are only talking about industrialized nations, so if the studies were only done in industrialized nations, that would be fine.
About Larkin: Larkin Robson is an LSAT Tutor and Attorney based out of New York City. He has a JD from NYU School of Law, has taught at West Virginia University School of Law and John Jay College in Manhattan. He has been teaching the LSAT for 8 years and founded an independent boutique LSAT company called 180 Degrees LSAT. Like Nathan, he scored a 179 on the actual test. His website is 180degreeslsat.com Tweet him @180degreeslsat.
From Larkin: I like the LSAT because there is actually a reason for the test. It is testing critical thinking and reading comprehension, both skills that are useful and valuable in the real world. I am a die hard philosophy buff and I think that the humanities and critical thinking are vastly undervalued in today's society, and the LSAT is essentially testing those skills. It means that studying for the test does not mean you are just studying for a stupid test, you are actually learning something useful.
Please ask questions and/or suggest corrections to anything that seems confusing... we want to make this the best resource we can for LSAT students. We'll have all the June 2013 explanations up as quickly as possible. Thanks for reading. Tell your friends! --nathan