We are asked to distinguish the role of the first sentence of the passage: "It is unreasonable to incarcerate anyone for any other reason than that he or she is a serious threat to the property or lives of other people."
The entire argument is trying to prove the truth of this first statement. It offers the reason that breaking the law does not justify incarceration since lawbreaking proceeds from two things: (1) ignorance of the law or the effects of one's actions (2) from the free choice on the part of the lawbreaker.
The argument further explains that the first aspect, i.e. ignorance, cannot justify incarceration and the second aspect, i.e. free choice, cannot justify incarceration since free choice proceeds from the desires od an agent, which are the products of genetics and environmental condition--and neither of those are controllable by the agent.
Thus, it is not reasonable to incarcerate anyone for any other reason than that he or she is a serious threat to the property or lives of other people. Meaning, the first sentence of the passage is the conclusion of the argument, i.e. answer choice (C): "It is offered as the main conclusion that the argument is designed to establish."
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