Choose the correct antonym for ”sullen” from the options given below.

The correct option is genial.

To find the correct antonym (opposite word) for sullen, let us analyze the meanings of the words involved:
• Sullen describes a person who is bad-tempered, gloomy, silent, and unsociable.
• Morose means sullen, gloomy, and ill-tempered. This is a synonym of sullen, not an antonym.
• Reticent means reserved, quiet, or hesitant to reveal one's thoughts or feelings.
• Timid means shy, nervous, or lacking courage.
• Genial means friendly, cheerful, warm, and extremely pleasant.

Comparing these definitions, we can see that a sullen (gloomy and unsociable) disposition is the direct opposite of a genial (cheerful and friendly) disposition. Therefore, genial is the correct antonym.