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.

Any matter that one finds novel or improvement over a conventionally used method supported by practical results, simulation, or theoretical construction deserves to be published as a research article (paper).

In your cases, the following applies.

• A's article could be a research paper provided her/his method is novel or unconventional.
• B's article could also be a research paper as such papers are available for life sciences as well as historical biographies.
• C's article can't be classified as a research paper although it can be recalled as a survey paper.
• D's article do not have sufficient qualities for a research or survey paper. It may be published as a news or magazine article.