Which word relates best to the quality of being lighthearted or lacking seriousness?

The word that best relates to the quality of being lighthearted or lacking seriousness is "levity." Levity denotes a lightness of mind, character, or behavior, often characterized by a lack of seriousness or an ability to treat serious matters in a cheerful or humorous way. It embodies a sense of frivolity and playfulness, making it suitable for describing a lighthearted attitude.

In contrast, the other choices convey meanings that are more serious or weighty. "Heavy" implies weight or seriousness, "gravity" refers to seriousness or importance, and "severity" suggests harshness or strictness. These terms are fundamentally at odds with the concept of lightheartedness, making them unsuitable in this context.