D is a programming language intended as an "improved" version of C++.
C was a successor to an earlier programming language called B, which was itself descended from BCPL. There was much debate over whether any successor language to C ought then to be called D (to continue in the alphabet) or P (to continue to take letters from the original BCPL). This was sidestepped by naming the successor language C++ instead (and some other languages, such as C# and Objective-C also took the tack of adding additional symbols or words to the name "C"), but eventually somebody did come out with a language named D. There doesn't seem to have been any language A, however.
D keeps many of the object-oriented features of C++ (but not quite all, so it's not fully compatible with that language), and adds such things as garbage collection and bounds checking.