Which word refers to an idea that is considered a fundamental belief?

The word "tenet" is the correct choice because it directly refers to a principle or belief that is held to be true, especially in a particular field of study or philosophy. It often denotes a foundational idea that guides the thought process or actions of a group, such as in religions, philosophies, or theories.

While "doctrine" also pertains to a set of beliefs or teachings—often in religious or ideological contexts—it typically implies a more formalized or systematic set of principles that may not necessarily be fundamental to all individuals or groups.

"Hypothesis" refers to a proposed explanation made based on limited evidence as a starting point for further investigation, which is not equivalent to a fundamental belief, as it requires testing and potential falsification.

"Thesis" generally indicates a statement or theory that is put forward as a premise to be maintained or proved, often in academic writing, and while it can reflect fundamental ideas, it lacks the broader applicability of "tenet" in denoting a widely accepted belief.

Thus, "tenet" is the most precise term for a fundamental belief shared among a community or discipline.