The correct answer is B. fact.
A theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment. A fact is a thing that is indisputably the case.
Option A, imagination, is the opposite of reality. It is the ability to form a mental image of something that is not actually present or that has not happened.
Option C, chance, is the possibility that something will happen or not happen. It is the absence of certainty.
Option D, thought, is the process of using one’s mind to form ideas or to solve problems.
In conclusion, the only option that is the opposite of a theory is a fact.