A well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses.

Here's why this is the best choice:

* Well-substantiated: Scientific theories are not just guesses. They are based on a large body of evidence and have been repeatedly tested and confirmed through experiments and observations.

* Explanation: Theories aim to explain why things happen, not just describe what happens. They provide a framework for understanding a phenomenon.

* Natural world: Scientific theories focus on the physical universe and its processes.

* Incorporates facts, laws, inferences, and tested hypotheses: Theories draw upon a wide range of scientific knowledge and integrate different pieces of information into a coherent whole.

Important Note: It's crucial to understand that scientific theories are not the same as "just a theory" in everyday language. In science, a theory is a very strong and well-supported explanation.