To determine the approximate straight-line distance between the cities of Darwin and Brisbane using the map's scale, you would need to visually measure the distance between the two cities on the map and compare that measurement to the scale provided. However, without the actual map or a clear representation of what the measured distance is on the map itself, I can't give an accurate approximation.
If you had taken the measurement from the map and found that this corresponds to the real-world distances shown in the options (2,500 km, 2,000 km, 1,800 km, or 1,500 km), you'd select the closest option based on your measurement.
Typically, the straight-line distance between Darwin and Brisbane is around 1,500 km. Therefore, if you measured the distance on the map and it aligned with the real-world distance, the correct answer would likely be:
1,500 km.