An antonym for the word "sullen" is "cheerful."
Explanation:
1. Sullen means gloomy, sad, or sulky.
2. An antonym is a word that has the opposite meaning.
3. Therefore, cheerful, which means happy, lively, or positive, is the antonym for sullen.
Example:
- If someone is feeling sullen, they may be quiet and withdrawn. In contrast, if someone is cheerful, they are likely to be smiling, energetic, and spreading positivity.