What is a restriction on visual observers while they are assisting a pilot?

The correct answer emphasizes that visual observers assisting a pilot must not operate a moving vehicle. This restriction is essential for maintaining safety and ensuring that the observer can fully concentrate on their duties, which include monitoring the airspace and ensuring the aircraft's safe operation. Operating a moving vehicle would distract the observer, possibly leading to oversight of critical visual cues necessary for the safe flight of the aircraft.

The other options do not align with the standards or guidelines for the roles of visual observers. Wearing a life vest is not always mandatory depending on the situation—it's typically required in certain water operations. Using personal electronic devices can also divert attention and compromise the observer’s ability to fulfill their role effectively. Finally, performing visual duties under any condition disregards the importance of situational awareness and the need for clear visibility; observers must work under suitable conditions to effectively monitor for potential hazards.