German.Steel,
What you're pointing out is exactly why answer choice (C) helps the argument. As you say, the author's argument is conditional on monetary compensation being possible. If monetary compensation is not possible, then the argument doesn't work very well. So it helps the author to say "The condition upon which your argument relied was, after all, true."
The "conditional" nature of the argument could be expressed this way: "As long as monetary compensation is possible, it will be adequate compensation. So, monetary compensation is adequate compensation." I don't think diagramming this is necessary, but it certainly works:
Premise: $ possible
$ adequate
Conclusion: $ adequate
Showing that the sufficient condition is true would thus make the necessary condition a good inference, which is precisely what we want.
Robert Carroll