The question deals with the geography of the aircraft routes. Each option lists a route from Perth to London via different cities in sequence.

Option 1 includes Perth, Bombay, Rome, and London. This route necessitates a considerable detour through India and over the Middle East.

Options 3 and 4, while involving different intermediate cities, also imply lengthier paths. Mombasa is located in Kenya, which is off the direct course to London.

Option 2, however, proposes the shortest air route from Perth to London, which is via Ankara and Paris. This route practically traces a straight line on the map, thereby minimizing the distance and thus the air travel time. This is achieved by travelling north-west from Perth to Ankara, then on to Paris, and finally to London.

However, to truly determine the shortest air route, a detailed study with exact coordinates of cities, altitudes, wind speeds etc., would typically be performed using specialist software.