These "boldface portion" questions are asking us to analyze the structure of the CR argument. These can be some of the trickiest CR questions, because the GMAT makes the structure difficult to analyze; they tend to be full of complicated twists and turns. When you're reading through the argument, it's crucial to break it down piece by piece. Here are the questions you should ask yourself:

1) What is the author's main point (i.e. the conclusion)?
2) What evidence is used to support that conclusion?
3) What is the author arguing against (i.e. the counterpoint)?

The first thing you should pinpoint is the conclusion. If you're unsure which sentence is the conclusion, ask yourself this - if I were paraphrasing, how would I sum up this argument? I might say something like "Scientists get less creative not because they're old, but because they've been there too long." That's pretty much what the last sentence is saying, so that must be our conclusion. Let's break down the other sentences accordingly.

"Scientists typically do their most creative work before the age of forty." Is this supporting or going against the conclusion? Well, it's tricky. This is a fact, so the author isn't arguing against it. But it's a fact that was used to support the viewpoint that the author is arguing against.

"It is commonly thought that this happens because aging by itself brings about a loss of creative capacity." Whenever you see language about what other people believe or used to believe (such as "it is commonly believed," "researchers have long thought," etc), it has to be a counterpoint. This is what the author is arguing against.

"However, studies show that of scientists who produce highly creative work beyond the age of forty, a disproportionately large number entered their field at an older age than is usual. " Whenever you see a transition word like "however," the author is shifting from a counterpoint to either his/her own conclusion, or the premises for that conclusion. So which one is this? Well for one thing, we've already pinpointed the conclusion. But just to be sure... conclusions are always opinions, and anything that starts with "studies show" must be a fact. So, this fact is being used to support our conclusion.

"Since by the age of forty the large majority of scientists have been working in their field for at least fifteen years," Here is another fact that is being used in support of our conclusion.

"the studies' finding strongly suggests that the real reason why scientists over forty rarely produce highly creative work is not that they have aged but rather that scientists over forty have generally spent too long in their field." As we've said, this is our conclusion. "Strongly suggests" and "the real reason" are two good clues that this is the most important point of the argument.

Now that you've deconstructed the argument, it's a good idea to write down what those boldfaced portions represent, so you'll know what to look for in the answer choices. We want an answer that says:
boldface 1 = evidence supporting the author's conclusion
boldface 2 = author's conclusion

When you're looking at the answer choices for these boldface questions, I find that it's really helpful to deal with just one portion at a time (kind of like reading vertically in SC). Look at what the answer choices all say about the first portion, then look at what they say about the second.

"(A)The first is a claim, the accuracy of which is at issue in the argument..."
Is the accuracy of that claim at issue? In other words, is the author debating whether it's true? No, the author is using it as support, so (A) is out.

"(B)The first is an objection that has been raised against a position defended in the argument..."
An objection raised against something the argument defends would mean a counterpoint ("the argument" really means "the conclusion of the argument"). This statement is defending the conclusion, not going against it. (B) is out.

"(C)The first is evidence that has been used to support an explanation that the argument challenges... "
Something that supports what the argument challenges would also be a counterpoint. (C) is out.

"(D)The first is evidence that has been used to support an explanation that the argument challenges..."
You'll notice that (D) starts out the same way (C) did, so we can quickly eliminate it, too.

"(E)The first provides evidence to support an explanation that the argument favors... "
Supporting what the argument favors means supporting the conclusion, which is what we're doing here!

(E) is the only one we haven't eliminated yet, but let's check to make sure the second portion matches, too. We're looking for something that says "conclusion."

"(E) ... the second is that explanation."
The explanation that the argument favors is the conclusion!

The answer is E.[/i]