Select the most appropriate option to fill in the blank.
Newton discovered the _______ of gravity while watching an apple fall from the tree.

The correct word to fill in the blank is principle, which means a fundamental truth or proposition that serves as the foundation for a system of belief or behavior or for a chain of reasoning (e.g., scientific laws/principles).
Belief means an acceptance that a statement is true or that something exists.

Thesis means a statement or theory that is put forward as a premise to be maintained or proved.

Code means a set of rules or a system of words, letters, or figures.