Of all the Reading Comprehension (RC) types, “main idea” questions are the most common. You can expect the GMAT to ask this question about almost every passage within the RC section. Identifying the main idea is the quintessential RC skill, so expect to practice it over and over again. It will help to read at a relaxed pace (2.5 minutes for a short RC passage, 3.5 for a long passage). It will help to practice taking notes. It will also help to practice repeatedly from reputable sources, like the Official GMAT Guide or the RC sections of the LSAT guides. As you study, always check the official answers each time and read the explanation, regardless of whether you got the question right or wrong. Doing this will allow you to think more like the test-writers. By practicing diligently, you’ll begin to notice how to weed out trap answers and how to select the best answer from the available options.
Typically, there will be only two answers that will seem like the right answer. So, even if you cannot narrow down to one, get to the Top 2. Once you are there, then check the word play and eliminate one.
1) Avoid answer choices that focus in on only one detail or paragraph of the passage, and do not express the main idea as a whole. Remember, the "main idea" of a passage carries throughout the entire passage. One paragraph might be a very detailed explanation of a model, theory, strategy, or scientific phenomenon, but you must ask yourself WHY the author includes these details, and how they fit in to the bigger picture. RaviChandra's example contains a great example of an answer choice that is far too narrow to be the main point:
2) Avoid answer choices that are overly extreme. Its rare that the main point of a passage is absolute, so choices that state things in absolute terms are almost never correct. RaviChandra's example also contains a great example of an extreme answer choice that is HIGHLY unlikely to be correct:
3) Avoid answer choices that distort details of the passage. These are the hardest to spot, but they are almost always there. They often include an extra reversal (saying that two things are similar, when the passage says they are different, like the example below), or include a word that sounds like something from the passage, but isn't actually there (if a whole passage is about "nuclear physics," any answer about "atoms," "science" or "physics" would not be correct).
4) Identify questions which use words from the passage, but distort the details given. When in hurry, you would be tempted to pick these answers, so be careful while reading