A vs. D

A tells us that government should be allowed to spend money on things whose intended goals don't benefit most people. While this would independently support the authors conclusion, it isn't the argument that he makes. The author makes an argument that regardless of the actual intended benefits, the unintended benefits actually do benefit most people. This is a pretty perfect match for D. The principle underlying the argument is something that describes the argument made, not an additional principle that would independently lead us to a similar conclusion.