According to the conclusion we’re trying to support, extremely wealthy people are extremely unhappy. (A) doesn’t get us anywhere close to that – it just says billionaires’ quest for “the highest level of happiness” has been impeded.

Like “Dang I’m so close to peak human happiness but my yacht needs repairs aw shucks guess I’ll have to settle for being super duper happy.”

(B) bridges the gap between the argument’s valid net effect reasoning and its conclusion about happiness. The premises establish that gaining wealth increases your number of unsatisfied desires. (B) tells us there’s an inverse correlation between your number of unsatisfied* desires and your happiness.

(B) is phrased inconveniently for sure. It’d be nice if it just said “The more unfulfilled desires you have, the less happy you are.” For correlational claims like this one, though, both phrasings are equivalent.

* unsatisfied == unfulfilled, btw

(C) tries to guard the argument against a possibility the author already acknowledges.

The columnist’s net effect reasoning makes ample room for satisfying desires to increase happiness. Their contention is that whatever happiness you gain from satisfying desires, if any, won’t outweigh the happiness you lose by creating a bunch of unsatisfied desires. The idea that satisfying desires could yield happy points is baked right into the logic.

If it turned out that satisfying desires did in fact increase happiness, that wouldn't weaken the argument (because the author would happily concede that point). It therefore wouldn’t strengthen the argument to say satisfying desires doesn't increase happiness.

Strictly speaking, (D) is neither here nor there because what people want isn’t necessarily what makes them happy. But if we assume people generally want stuff that makes them happier (which really isn’t a crazy assumption), (D)’s suggestion that the vast majority of wealthy people want to get wealthier would actually weaken the argument.

The columnist wants to establish a firm link between wealth, unfulfilled desires, and unhappiness. Introducing additional factors that could affect happiness muddies the waters, weakening the author’s position.