Which term best describes an unstoppable or relentless force?

The term that best describes an unstoppable or relentless force is "inexorable." This word conveys the idea of something that cannot be stopped, changed, or influenced by any means; it is unyielding and persistent. In literature and conversation, it often refers to forces such as fate, time, or nature, which continue onward without regard to human desires or actions.

The other terms have meanings that do not capture the notion of being unstoppable. For instance, "ameliorate" refers to making something better or improving a situation, which is quite the opposite of an unstoppable force. "Proselytize" means to convert someone from one belief to another, which involves persuasion rather than an unstoppable nature. "Diminutive" describes something that is small or minor, lacking the robust force implied by "inexorable." Therefore, when discussing an unstoppable force, "inexorable" is the appropriate term to use.