Explanation:

- The original sentence uses "Unless," which is a conditional word that sets a requirement for the action of traveling by air.

- Option 1: Provided – "Provided" means the condition under which something occurs, suitable for similar contexts but implies the condition is being actively offered.

- Option 2: Only – "Only" restricts or limits but isn't conditional; it wouldn’t fit smoothly in this conditional requirement context.

- Option 3: Until – "Until" relates to time and is not fitting for expressing preconditions for an action.

- Option 4: No improvement required – "Unless" is already an appropriate word to use for this type of condition.

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