Which of the following is a theory?
A) an educated guess.
B) a very specific prediction.
C) observations made during the voyage of the Beagle.
D) a broad explanation that has been scientifically tested and supported.
The correct answer is D - a theory is a broad explanation that has been scientifically tested and supported. Possible answer A could be a definition for a hypothesis. Possible answer B does not allow the wide scope of information that can be included in a theory, and possible answer C does not address the explanatory function served by a theory. A theory may serve as the starting point for observations as well as providing a foundation for experimentation and drawing conclusions. The nature of much of scientific research, however, is such that absolutely final and irrefutable conclusions are often not possible. Rather than being able to state that "this is an irrefutable fact for all time," scientists more often conclude "the evidence available to this point supports this theory" - pending revision if necessary based on new evidence.
D is correct. A is incorrect because an educated guess is a hypothesis. C is a ridiculous answer. There is no formal term for a very specific prediction. A theory, by definition is a plausible or scientifically acceptable general principle or body of principles offered to explain phenomena. These are generally proven by multiple tests.