A theory is a list of statements that are (more or less) proven facts that are strung together to form an explanation for a phenomenon. A theory is assumed or accepted as true until something disproves it. A theory is falsifiable, meaning that something can disprove it at any point in time. But, a good theorist will tweak his or her theory a bit to account for the falsified data.
Long story, short, this question is worded weirdly, because all theories are the above definition. An educated guess in everyday life isn’t a theory; it is a hypothesis. Secondly, the most basic definition of theory is “a well-supported explanation that is accepted as true until it is falsified,” so the answer can be both A and D. If I had to pick one though, I would go with D.