The method is probably the oldest form of research in existence. questioning secondary research observational interpretive

I would say that the best answer to the question: The method is probably the oldest form of research in existence, would be: Observational.

Explanation:

Although historically it is difficult to pinpoint the exact way in which humans began to research, and especially to develop methods of research, it is known that the curiosity to understand what was going on around their environment, was what led humans to observe, and then question what was happening. This, then further led people to design and develop methods that allowed them to answer those questions more accurately and efficiently. But the first step for this to happen, which is probably why it is one of the oldest forms of research, is observation, people in the middle of the field, observing and then questioning themselves, in order to find answers.