The answer is C - an explanation based on observation, experimentation, and reasoning.
The reason why the correct answer is C and not D is because D would most likely be a hypothesis. Each hypothesis corresponds to an idea that needs empirical testing and we want to see if it's true or not.
A theory on the other hand is a general collection of possibly multiple hypothesis that have been tested and confirmed. A theory incorporates a more conclusive framework under which scientists can test their hypothesis.