When a “flaw” answer to an LSAT logical reasoning question contains words like “takes for granted” or “presumes, without justification,” you are dealing with an assumption of some kind. When you assume something that is necessary to the argument, you are definitely engaging in flawed reasoning. (As the old saying goes, “When you assume, you make an ass of u and me.”) Not every assumption is a necessary assumption, however, so the challenge is to discern whether this particular assumption is essential to this argument.
If an assumption is truly necessary, the argument self-destructs if you assume the logical opposite. Thus, for example, if I say, “Tuna is Marcia’s favorite lunch, so she is going to love my solid white albacore salad,” you have to assume that “albacore” is a kind of tuna. If you assume that albacore is not a type of tuna, the whole argument falls apart. You can always determine whether an assumption is truly necessary by seeing what happens when you assume its opposite.
This is why “extreme” statements are so seldom truly necessary assumptions. If I make an extreme assumption (for example, “solid white albacore is the only type of tuna”), then the logical opposite of that assumption generally won’t contradict the conclusion. Thus, if I assume that there other types of tuna besides albacore, my wife may still love her lunch.
That doesn’t mean that extreme answers are always wrong, however! You should check an extreme answer by assuming the logical opposite, and then see what effect that has on the argument. If an extreme answer is the right answer, assuming the opposite makes the argument fall apart. If it is the wrong answer, the opposite assumption has no real impact on the argument as a whole.
(It takes a little training to properly negate a logical statement. The logical opposite of “white” is not “black,” as you might suppose, but “not white.” Thus, pink, green, or tangerine are equally the opposite of white (in a logical sense). 7Sage.com has this post on how to find the logical opposite.)