Explanation:

- "On cloud nine" is an idiom used to describe a state of extreme happiness and joy. It's like feeling you're floating on clouds because everything is going so well.

- Option 1: Extremely happy

- This is the correct meaning. Feeling ecstatic or overjoyed perfectly fits the idiom's intent.

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- Option 2: Flying with a parachute

- This is not related to the idiom. It describes a physical activity, not an emotional state.

- Option 3: Frequently flying by air

- This suggests someone travels often, but it doesn’t convey the emotional state described by the idiom.

- Option 4: Extremely sad

- This is the opposite of what the idiom means. "On cloud nine" suggests happiness, not sadness.