23. (C)
Question Type: Assumption
Before we even get to the answer choices, we can spot a gap in this argument. An action is wrong if, and only if, it reduces the aggregate well-being of the people affected by it. The author concludes that any action that does NOT reduce the aggregate well-being is by default right. The author assumes that there are only two kinds of actions: right and wrong. Answer choice (C) expresses this assumption in clear terms.
(A) is incorrect. We’re interested in actions that leave the aggregate well-being unchanged. What do we call these actions?
(B) is tempting, but it misses the issue. We don’t care if an action can be both right and wrong. The issue is whether it can be neither.
(D) misses the issue as well. We don’t care if there are such actions, but rather what to call them (right, wrong, or something else) if they did exist.
(E) is out scope. "Consequences" in general is a concept that is beyond the scope of the argument.
#officialexplanation