what is a central argument?

Answer 1

It is the most important point made.

For example, the central argument of the Declaration of Independence is that all men have rights that are inalienable.

Answer 2

In order to answer your question, I will refer to Stone et al. (p. 19), who observe that "the two main forms of nonfiction are exposition (= explaining something) and argument (urging a course of action or a point of view"). And the authors add that "the argument [= evidence for or against] depends upon evidence [the explanation], and exposition is the form evidence usually takes" (p. 22).

A "central argument" is the thesis (= the main proposition), the proposition to be proved (for example, "Pluto is not a planet"). The thesis is equivalent to the "research question" (" Is Pluto a planet?"). Therefore, in research papers, writers state either the central argument (which must be a statement) or the research question (which must be a question). As far as content, though, both are the same.