In regard to fixed-wing airplanes, which is a recommended caution?

Touching the propeller when shut down presents a significant risk of injury, which is why it is recommended to exercise extreme caution around it. Even when an engine is shut down, the propeller can still move unexpectedly due to residual engine motion or inadvertent activation of the ignition system. Ensuring that personnel are aware of this risk can prevent potential accidents and injuries on the tarmac.

The other options, while they touch on safety, do not carry the same level of inherent risk. Active anti-torque pedals are specific to rotorcraft, making that option not applicable in the context of fixed-wing aircraft. While there are usually designated areas on wings that are considered walkable, not all parts are safe, so caution must also be observed. Items within the cockpit can be a distraction, and safety protocols exist to ensure they are handled properly, but they do not pose the same immediate physical danger as the propeller does. Therefore, the mention of caution regarding touching the propeller when shut down is the most relevant and critical safety reminder in this context.