Which word describes something that is "settled and quiet"?

The term that describes something that is "settled and quiet" is indeed best captured by the word "sedate." This word conveys a sense of calmness and tranquility, often associated with a deliberate and composed demeanor. When something is sedate, it implies a lack of agitation or movement, thus aligning perfectly with the idea of being settled and quiet.

In contrast, the other options represent different characteristics. "Implacable" describes something that cannot be appeased or settled, often linked to relentless or unforgiving attitudes. "Infinitesimal" refers to something extremely small or minuscule, which does not convey the desired sense of calmness or tranquility. "Stentorian" denotes a loud or powerful voice, emphasizing volume rather than a serene or quiet state. Therefore, "sedate" is the most appropriate choice to convey the desired meaning of being settled and quiet.