The value of a society must be judged on the total amount of

The evidence that that the author provides is his supposition about satisfaction being the best measure of society; his conclusion is that an absolutely free society is the best one.

Whenever we see a shift in terminology like this, we should instantly recognize a classic Scope Shift error. The author's evidence is about satisfaction, but his conclusion is about opportunities to unleash individual desire. His assumption must bridge that gap. So, we make our prediction (Kaplan recommends always predicting!): "Citizens free to fulfill their desires have the most satisfaction"

(D) matches perfectly.

The best thing about spotting the scope shift, and making this prediction, is that you won't be tempted by the very attractive (A)--an answer so misleading that we accidentally marked it as the OA! Fortunately, our brand-new 2010 GMAT course includes a revision of the few errors floating in our system, but I'm digressing. (A)'s problem is the phrase "never". This make it too extreme to be a necessary assumption.

It may be a bit tricky to grasp why on first glance; on Test Day, you want to simply pick (D) because it matches our prediction and move on. But if you're really stuck, or if you want to understand this for study purposes, the best way to think about it is with Kaplan's Denial Test. The question stem is asking us for the answer that must be true for the argument to hold; so if we deny (state the opposite of) the correct answer choice, then the argument should fall apart. On the other hand, negating a false answer should have no effect on the validity of the argument. Remember, the opposite of "never" isn't "always." It's "not never," or "at least one." So, the opposite of (D) is "People's desires are sometime incompatible, and there is at least one person's desires exclude the gratification of another's."

All right, so we've got Bob and Joe living in Egopotamia. Both want to marry Mary--and only one will get his way. The other will be left sad.
Meanwhile, the other 100 Million Egopotamian citizens get to do exactly what they want.
Could Egopotamia be the best society? Certainly! Two unhappy citizens don't somehow negate the million of other satisfied ones. In other words, (A) can be made false, and the author's conclusion could easily still be the case. (A) therefore cannot be said to be a necessary assumption, and is therefore incorrect.