Explanation:

- On cloud nine: This idiom means feeling extremely happy or elated. Imagine floating high up on a cloud due to excitement or joy.

- Option 1, Good number: This isn't the meaning of the idiom. "On cloud nine" isn't about numbers at all.

- Option 2, Travel by airplane: Though being on a plane involves being up in the sky, it's unrelated to the idiom. It's about feelings, not travel.

- Option 3, Extremely happy: This is the correct meaning. When someone is on cloud nine, they're very happy or delighted.

- Option 4, Heavy rain: This has no connection to the idiom. "On cloud nine" describes emotions, unlike weather conditions.