This is a Strengthen Principle question. Let's evaluate what the stimulus is saying.
The conclusion is: "It follows that the theft was specifically carried out to suit the taste of some individual collector for whose private collection the pieces were destined." Why? Because the criterion for selection was not greatest market value.
Thus, based on the evidence of the criterion for selection of art taken and left alone, the author has concluded the type of thief and reason for his theft. This is exactly what answer choice (C) states: The pattern of works taken and works left alone can sometimes distinguish one type of art theft from another.
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