When do true airspeed and groundspeed increase proportionally?

True airspeed and groundspeed increase proportionally as the density altitude increases. As density altitude rises, the aircraft's performance characteristics change, particularly in relation to lift and drag. In high-density altitude conditions, the air is less dense, which results in reduced engine performance and lower aerodynamic efficiency at a given indicated airspeed.

When operating at higher density altitudes, true airspeed, which is the speed of the aircraft relative to the surrounding air, becomes increasingly significant. As groundspeed, which is the speed relative to the ground, is influenced by wind conditions, true airspeed will directly affect how quickly the aircraft is moving over the ground.

In this situation, if the aircraft increases its true airspeed, it will also increase its groundspeed, especially when flying in favorable wind conditions. Hence, with an increase in density altitude, true airspeed and groundspeed can be expected to rise in a proportional manner, making the option about increasing density altitude the correct choice for this scenario.

If the density altitude were to decrease or remain the same, other factors would influence the airspeed differently, potentially leading to a less predictable relationship between true airspeed and groundspeed.