You will know that you are looking at one of these questions because you will see an underlined sentence in the passage, and the question will always read exactly: "Which choice best describes the function of the underlined portion in the text as a whole?"
Note that this is not simply asking for the meaning of the underlined text. It's asking what function it plays in the passage as a whole. Sometimes the right answer will just be a paraphrase of what is conveyed in that sentence, but often it will be more about how that sentence relates to the other sentences in the passage, rather than what the sentence actually says. For example, the right answer might be something like "It presents a claim that is then refuted with a counterexample." This is why it's important to get a feel for the passage as a whole, and think about how the underlined text relates to the other sentences. Does it make a claim that is then supported by the next sentence? Is it an example of the sentence that comes before it? These are the kinds of things you should be looking for. Before looking at the answer choices, try to think of how the underlined text plays into the passage as a whole.
Be on the lookout for all the common wrong answer types we discussed in
Wrong Answers. One type of wrong answer to look out for in particular is an answer that is supported by the text, but doesn't relate to the underlined portion.