What adjective best describes a person's attitude that lacks seriousness or respect?

The term that most accurately describes a person's attitude that lacks seriousness or respect is "flippant." This adjective conveys a sense of being frivolous or dismissive, often in situations where a more serious or respectful demeanor would be expected. For example, a flippant remark in a serious discussion can come off as insensitive or trivializing the topic, suggesting a lack of concern for the matter at hand.

In contrast, the other options do not align with the context of lacking seriousness or respect. "Apathetic" refers to a lack of interest or concern, but it doesn't inherently include the disrespectful or dismissive connotation found in "flippant." "Ebullient" describes someone who is enthusiastic and full of energy, which is quite the opposite of lacking seriousness. "Lethargic" indicates a state of sluggishness or lack of energy and is unrelated to attitudes of seriousness or respect. Thus, "flippant" is the most suitable choice for describing a casual or disrespectful stance.