The word theory used in a scientific sense most closely means
a. that of which a scientist is most certain, based on previous knowledge.
b. a guess made with very little knowledge to support it.
c. an explanation based on observation, experimentation and reasoning.
d. an explanation that is based only on observations and can be tested.

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.