keyword "clearly" separates the conclusion from the premise.

Paraphrase - People get tired of watching the VCR after 6 months of purchase. The sales which is high now will slacken in the coming days. Clearly, the market for VCR is going to face hard times when the market is saturated.

"Weaken" means weaken the conclusion.

(D) The early buyers of a novel product are always people who are quick to acquire novelties, but also often as quick to tire of them. -> this tells us that there will ALWAYS be a market for the product and hence weakens the conclusion.

Sure, but the passage or (D) does not indicate that the early buyers are not bored anymore and so will continue to purchase videos, which will weaken the conclusion. Passage suggests that early buyers are bored and once all buyers are bored, the businesses will fail. (D) explains the buying patern of early buyers but doesn't say why this set of people will buy videos again (after all they are all bored and why would you buy videos if you are bored).
Maybe I'm misinterpreting this a bit but this is what put me off a little "lost interest in obtaining videos to watch on it". This suggests that early buyers don't care about buying or renting videos after they are bored. If eventually ALL people end up this way, we will have businesses that are not selling or renting any videos. Nowhere does it suggest that early buyers will keep buying videos (and continue their buying habbits) after they are bored. Am I missing something?

Six months or so after getting a video recorder, many early buyers apparently lost interest in obtaining videos to watch on it. The trade of businesses selling and renting videos is still buoyant, because the number of homes with video recorders is still growing. But clearly, once the market for video recorders is saturated, businesses distributing videos face hard times.

Which of the following, if true, would most seriously weaken the conclusion above?

(A) The market for video recorders would not be considered saturated until there was one in 80 percent of homes.
-->this is an irrelevant fact
(B) Among the items handled by video distributors are many films specifically produced as video features.
-->irrelevant
(C) Few of the early buyers of video recorders raised any complaints about performance aspects of the new product.
-->that's good to know.. eliminate..what does this have to do with saturation and low levels of video sales in the future?
(D) The early buyers of a novel product are always people who are quick to acquire novelties, but also often as quick to tire of them.
-->a possible reason why videos still may do well even after the market is saturated. If other people continue to buy videos after 6 months the businesses distributing videos may not face hard times
(E) In a shrinking market, competition always intensifies and marginal businesses fail
-->good to know .. but this is not relevant...besides ...video recorders and video distribution are not competitors .. rather they benefit from each other.