Which of the following situations requires attention to wingtip vortices?

The situation requiring attention to wingtip vortices is during takeoff or landing at busy airports. Wingtip vortices are a byproduct of lift generated by an aircraft's wings, and they tend to be particularly strong and hazardous during these critical phases of flight, especially when aircraft are operating close together in busy airspace.

When taking off or landing, aircraft fly at lower altitudes and speeds, and they may be in proximity to each other, increasing the risk of wake turbulence encounters. Larger aircraft produce stronger vortices due to their greater mass and lift, and these vortices can remain in the air for several moments after the aircraft has passed. Therefore, pilots must be aware of their flight paths in relation to preceding aircraft, especially if they are larger, to avoid loss of control due to unexpected turbulence.

In contrast, the other scenarios may involve other flight dynamics or challenges but are not specifically related to the heightened risk posed by wingtip vortices as seen in crowded airport operations.