Explanation:

- Option 1: has

- The phrase would be "The mad has the world of their own."

- This is grammatically incorrect because "the mad" implies a plural noun, requiring a plural verb.

- Option 2: had

- The phrase would be "The mad had the world of their own."

- This suggests possession in the past, which can be contextually correct but is unusual for a general statement.

- Option 3: is

- The phrase would be "The mad is the world of their own."

- This is incorrect because it lacks logic and plural agreement.

- Option 4: have

- The phrase would be "The mad have the world of their own."

- This is correct; it represents a current, general situation involving a plural subject.