The correct answer to the question: Using our context clues, the theory of a central fire most likely means, would be, B: That a fire must exist in the center of all things.
This belief of a Central Fire, comes from the astronomycal system born, in part, from the philosophy of Pythagorean philosopher Philolaus, in the 5th century BC, and it was the ruling belief for quite some time. This system established that the Earth, the Sun, and other celestial bodies, gravitated around an unseen Central Fire. This is why the correct answer here is B. Because the Greeks at the time believed that everything gravitated around this unknown, and unseen “Central Fire”.