The greatest vortex strength occurs when the generating aircraft is:

The correct answer is based on the principles of wake turbulence generated by aircraft. Vortex strength is influenced primarily by the weight, configuration, and speed of the aircraft.

When an aircraft is heavy, it generates stronger vortices due to the increased lift needed to support its weight. Being "clean" refers to the aerodynamic configuration of the aircraft, typically meaning that landing gear are retracted and flaps are up, which minimizes drag and allows for more efficient lift generation. When an aircraft flies slowly, the vortices created are allowed to linger longer in the surrounding air because they don't have as much forward momentum to dissipate quickly. Thus, when an aircraft is heavy, clean, and slow, the combination of these three factors leads to the greatest strength of the vortices it generates.

In contrast, other configurations either mitigate vortex strength or accelerate the dissipation of the wake turbulence. For instance, a light aircraft generally generates weaker vortices, and a dirty configuration (landing gear extended, flaps down) tends to create more drag and reduce lift efficiency, leading to weaker vortices. Fast speeds can also help in dispersing the vortices more quickly, reducing their strength.