What does the term "sullen" best describe?

The term "sullen" is best associated with a mood or demeanor that is sad, sulky, or showing a brooding ill humor. It typically describes someone who is silent and withdrawn due to sadness or ill temper. The word carries an implication of a gloomy or morose attitude, often marked by a lack of enthusiasm or a heavy silence.

In contrast, the other options present descriptions that are quite the opposite of "sullen." For instance, joyfulness, brightness, excitement, cheerfulness, and liveliness all suggest positive, upbeat emotional states, which do not align with the heavy, brooding nature implied by "sullen." Thus, "sad and sulky" is the only choice that fits with the emotional weight and connotation that the term "sullen" conveys.