Which word best describes a lighthearted manner?

The term "levity" perfectly encapsulates a lighthearted manner because it refers to a quality of being cheerful, carefree, or buoyant in attitude. Levity implies a sense of lightness and humor, often in situations where one might expect seriousness. This attitude fosters an atmosphere of ease and relaxation, making it fitting for descriptions of merry or spirited interactions.

In contrast, the other options convey a tone or mood that directly opposes lightheartedness. "Serious" suggests a grave or solemn approach, while "somber" conveys a mood that is dark and serious, often associated with sadness or reflection. "Melancholy" describes a deep, persistent sadness or sorrow, which is far removed from a carefree disposition. Thus, "levity" stands out as the most appropriate choice.