What exactly is a research paper?
A research paper is a paper that contributes some new knowledge to your field. That is, if you do some work (research) that adds to the body of knowledge in your field, and you write up the methods and results of your research in a paper, this can be called a research paper.

So in your example cases:


If A's technique is an important bit of knowledge that will move the field forward, then a paper describing it is a research paper. If it's a straightforward application of things that are already known, or a hack that's not going to move the field forward, it's probably not going to become a research paper.
D's work would not be considered research, if it's obvious to those in the field.
I don't know enough about standards in history to judge what's considered a contribution to the field, so I can't comment on B or C.