What best describes an assumption?

An assumption is fundamentally a statement or idea that is accepted as true without requiring proof or verification. This definition aligns perfectly with the selected answer, which highlights the notion of acceptance of a belief or premise that may not be substantiated by direct evidence. Assumptions are often taken for granted in reasoning processes or arguments, as they serve as the foundational beliefs upon which conclusions are drawn or arguments are built.

In contrast, the other options present different concepts. A well-supported conclusion based on evidence implies that there is substantiation behind the conclusion, which contradicts the very nature of an assumption. An argument based on factual data also denotes a reliance on evidence, rather than unproven belief. Meanwhile, a theory that has been widely tested suggests a framework that has undergone significant scrutiny and validation, which is again at odds with the idea of an assumption being accepted without proof. Thus, the focus on believing something to be true without the need for proof is what makes the chosen description of an assumption the most accurate.