If a story took place on another planet, it would most likely be a A. science fiction book. B. romance novel. C. historical fiction. D. mystery.

A. science fiction book.

Explanation:

A story taking place in another planet is most likely a science fiction book for two simple facts:

We do not know for sure if there are other inhabited planets in the universe. We do not have space travel powerful enough to allow us to conquer and inhabit planets other than Earth.

Thus, in order to set a story in another planet, we need to assume that at least one of the two statements above does not hold true, which requires fictional assumptions of scientific character, the very definition of science fiction.