Explanation:

- Jet planes create trails, commonly known as contrails.

- These trails form when the water vapor in the exhaust from jet airplanes condenses into ice crystals.

- Option 2: Water vapours

- Water vapor in the exhaust freezes to form trails.

- Cold, humid air at high altitudes helps in this formation.

- Carbon dioxide, carbon monoxide, and hydrogen are also found in exhaust emissions, but they do not directly create trails in the sky.

- Option 1: Carbon dioxide

- This is a greenhouse gas, not linked to trail formation.

- Option 3: Carbon monoxide

- A pollutant rather than a trail-forming substance.

- Option 4: Hydrogen

- Present in emissions but does not impact trail visibility.