Explanation:

- The speed of an aeroplane is calculated using the formula: Speed = Distance / Time.

- Given distance: 900 km.

- Given time: 1.5 hours.

- Calculating speed: 900 km divided by 1.5 hours = 600 km/h.

- Therefore, the speed of the aeroplane is 600 km/h.

Option 1: 500 km/h

- This is less than the calculated speed of 600 km/h.

Option 2: 300 km/h

- This option is much lower than the calculated speed.

Option 3: 600 km/h

- This matches the calculated speed and is the correct choice.

Option 4: 400 km/h

- This is also lower than the calculated speed.