Section V shows the ride from the airport to Lucy's aunt's house.
The first thing to consider is that cars are slower than airplanes, so cars' slopes will be less steep than airplane slopes. Section I represents the ride from Lucy's house to the airport. II is how long she waits to board, and III is the plane ride. IV is the amount of time that Lucy waits to be picked up. That leaves section V, which is the car ride to her aunt's house.
So, using logic and the steepness of the slopes, we can conclude that section V represents the ride from the airport to Lucy's aunt's house.