There's actually not much to diagram here, but here's one way of doing it. We can diagram the premise as
Employers - Many - > Treat employees fairly
And the conclusion in this case is simply: Using others as a means to one's own ends is not always morally reprehensible or harmful to others.
The assumption here we need and that is provided in (C) can be diagrammed as:
Employers - > Use employees as a means to their own ends
So employers use employees as a means to their own ends, but many treat them fairly, and so using employees as a means to one's own ends isn't always morally reprehensible or harmful to others. Hope this helps!