Which of these terms is associated with expressing a serious or respectful attitude?

The term associated with expressing a serious or respectful attitude is "pallid." In this context, "pallid" can denote a lack of vibrancy, often implying a subdued or serious demeanor. While it may typically describe something as pale or lacking in color, when applied to attitude, it suggests a tone that is restrained or somber, which aligns with seriousness and respect.

The other terms do not convey the same sense of respect or seriousness. For example, "flippant" refers to a lack of seriousness, often in a disrespectful manner, which is the opposite of what is intended here. "Apathetic" indicates a lack of interest or emotion, failing to represent any serious or respectful attitude. "Ebullient," on the other hand, describes someone who is cheerful and full of energy, which would contradict a serious demeanor. Thus, "pallid" emerges as the term that embodies a more serious or respectful attitude compared to the others.