Funny how much is just personal preference...
As an artist, I don't mind when people stop responding after prices are given because just about any way they respond besides "sign me up!" tends to lean toward insulting. Less is more, say "thanks" or whatever but don't offer excuses because we both know they're lame, yeah?
...But back to the topic - I prefer D in terms of politeness, but C if they're really serious. Anything that makes me have to contemplate pricing and how much I *really* want to keep it and whatnot has less chance of a response, or at least a timely response, because I am horrible at decision making. C is less effort on my part, I just have to decide if that number is worth it and I'll let you know.