A is the only right answer. Something that is not due is undue. Some people misspell "undue" as "undo," but how does undoing something make any sense in that context? It doesn't.

The others:

• B features "wets my appetite." That should be "whets." I suppose your mouth could be wet if you drool before you eat, but your appetite itself is kind of more of a concept, isn't it? It can't get wet. "Whetting" refers to increasing or making keener.
• C features the commonly misunderstood phrase "free reign." In fact, this should be "free rein," as in the contractors in question are not tethered by reins of any kind. "Reign" makes sense because it seems that if someone was free to reign, the person would be in charge, but that is not what the people who coined the phrase meant when they did so.
• D features a replacement of "route" for "root." I haven't seen many people who are actually good at language mess this one up, but it's always possible. "Route" is a planned path. "Root" is to cheer or support (well, or part of a tree, or a type of soda).