I put a new thermostat in my truck today and i now have a fuel leak, i attached a photo and it would be helpful if you could tell me what line A and C are and where they come from, and what the valve (B) does. My fuel leak is coming from line D

 

What you have picture of there is the water drain.

It comes from the filter manager at the back of the engine and drains water off through the line that goes down by the front of the engine.

With the engine off, loosen the valve and then tighten it up again. They are known to leak slightly as they age. You may have to snug it down with a set of pliers. Do not overtighten, just a bit more than hand tight.

Another possibility is the line is split from the Filter manager to the valve. Replacement is the cure. Not a big deal, it's just oil rated hose pushed on and clamped.