Which term would you use to describe something that has an unyielding nature?

The term that best describes something with an unyielding nature is "refractory." This word is often used to refer to materials or substances that resist being shaped or changed, particularly under heat or other forms of pressure. In a broader context, it can also describe attitudes or behaviors that are stubborn and resistant to influence or change. The quality of being unyielding is intrinsic to the definition of refractory, aligning perfectly with the question.

In contrast, the other options embody qualities that are opposite or unrelated to unyielding. "Fluid" suggests adaptability and a capacity to change readily under various circumstances. "Axiomatic" refers to statements that are self-evident; it doesn't imply resistance but rather a foundational certainty. "Flexible" denotes the ability to bend or adjust, which is the antithesis of being unyielding. Thus, "refractory" is the most appropriate choice to signify an unyielding nature.