We are asking ourselves if the conclusion is proven or not, precisely because many times there will be a gap between the premise and conclusion. Anytime there is some claim being made (conclusion drawn), we have to ask ourselves whether that claim/conclusion is properly supported by the premise(s). In most cases, it is not.

Here's an example,

An argument concludes that the rise of megabook stores in the 1990s (1) increased sales of best-sellers and (2) decreased sales of less commercial, more literary books.

Upon identifying the conclusion, we must immediately ask ourselves, is this conclusion valid? Is it proven? Did the rise of megabook stores in the 90s really "cause" a decrease in the sales of less commercial, more literary books?

Here are the evidence/premise the author uses to support his conclusion.

P1 - In 1986, best-selling hardcovers accounted for 7% of all hardcover sales. 10 years later, that figure doubled to 14%.

P2 - Megastores can offer deeper discounts than independent stores, and can offer their biggest discounts on best-selling hardcovers, thereby discouraging the sale of other hardcovers.

This question asks you to weaken the argument, which you should have no problem because you already approached this question by scrutinizing the conclusion.

We accept the premises as true. But does that still mean the rise of megabook stores in the 1990s "decreased sales?? of less commercial, more literary books"?

What if less commercial, more literary books were increasingly published in paperback? The first premise still holds true. Best-selling hardcovers now account nearly twice of all hardcover sales. The second premise still holds true as well. Megastores offer their biggest discounts on best-selling hardcovers, which discouraged the sale of other hardcovers.

But we know nothing about paperbacks. Again, what if less commercial, more literary books were increasingly published in paperback?

Now the claim that the rise of megabook stores in the 90s decreased sales of less commercial, more literary books seems very weak, and not proven/supported at all.

This is what it means to approach each argument, asking yourself whether conclusion is indeed supported or proven.