Correct Answer: Formal

Solution : The correct choice is the fourth option.

Explanation:

Colloquial refers to language or style used in ordinary or informal conversation rather than formal speech or writing. The antonym formal represents a manner of communication that adheres to established rules, conventions, or standards.

The meanings of the other options are as follows:

• Informal means relaxed, and casual.
• Slack means loose or negligent.
• Slang is informal language or expressions used within a particular group.