Correct Answer: Serious

Solution : The correct choice is the first option.

Explanation:
Witty refers to someone who is clever and humorous. Its antonym, serious , denotes someone who is solemn or lacking humour.

The meanings of the other words are as follows:

• Cynical : It means distrusting others' motives, especially seeing the worst in people or events.
• Callous : It refers to being emotionally insensitive or showing a lack of empathy.
• Giddy : It describes a sensation of whirling and a tendency to fall or stagger.