Which of the following best defines a theory

A)  explanation of unobservable phenomena 
B)  related speculations that are based on common sense 
C)  system of interrelated ideas used to explain some observation 
D)  string of unrelated observations

A theory is a system or set of interrelated ideas used to explain some observation or phenomenon. It is supported by evidence and can be tested through experimentation. Option A is partly correct but does not encompass the broad scope of a theory. Option B is incorrect as it implies that theories are based solely on common sense without sufficient empirical evidence or testing. Option D is entirely incorrect as a theory consists of related observations and ideas, not unrelated ones.