The argument provides us with two premises about a major art theft. First, the argument points out that the pieces were "carefully selected." Then, it points out that the "criterion for selection" was not "greatest estimated market value": in other words, the pieces stolen weren't the most valuable, in terms of market price. From these two premises, the author concludes that the theft was "specifically carried out" according to the taste of one individual collector who wanted to add these pieces to their collection.

There is a gap in the argument between the premises and the conclusion. The premises tell us, first, that whoever committed the theft was clearly making deliberate selections, and second, that they were apparently not choosing based off market price. These premises would strongly support the conclusion that whoever committed the theft was motivated by something other than greatest estimated market value. But the argument draws a much more specific conclusion about the motivations of the crime: the crime was specifically driven by the tastes of some individual collector.

The argument never establishes how we get from the premises to the conclusion that it must be one individual collector, as opposed to any of the other possibilities: several collectors, a crime ring focused on rare (but not necessarily the most valuable) artwork, etc. If this were a Strengthen or Sufficient Assumption question, this would be the gap we would want to fill. But notice that the question stem doesn't ask us to make the argument stronger, or to guarantee the conclusion: i.e., to add support to the argument stimulus. It asks us to identify what principle is already being relied on by the argument, so we'll need to look at the answer choices and see which one of them is best supported by the stimulus.