Which of the following describes someone who appears coolly unconcerned?

The term that best describes someone who appears coolly unconcerned is "nonchalant." This word captures the essence of a calm demeanor that suggests a lack of anxiety or interest in a situation. A nonchalant person typically exhibits a relaxed posture, unhurried speech, and a general air of indifference, making them seem unfazed by circumstances that might provoke a more emotional or concerned response in others.

In contrast, terms like "penitent" refer to someone feeling remorse or regret, which inherently involves concern and emotional heaviness. "Pliant" describes someone who is easily bent or influenced, not necessarily reflecting a calm or indifferent attitude. Lastly, "ponderous" implies something that is heavy, both in weight and in seriousness, suggesting a lack of lightness or ease that contradicts the idea of being unconcerned. Therefore, "nonchalant" accurately conveys the notion of appearing coolly unconcerned.