How far must the black disk be visible from the pool deck?

The correct answer indicates that the black disk must be visible from the pool deck at a distance of 9 meters. This requirement is important for safety and visibility during aquatic activities. The black disk is typically used as a reference point to help lifeguards and staff monitor conditions in the pool, ensuring that swimmers can be effectively observed and that the area remains safe.

Visibility at this distance allows lifeguards to maintain a clear line of sight to monitor swimmers effectively and respond quickly in case of an emergency. Ensuring that the disk can be seen at 9 meters not only facilitates better supervision but also contributes to overall safety protocols in the aquatic environment. The choice of this specific distance likely stems from established safety standards within aquatics, aiming to maximize the effectiveness of supervision while minimizing the risk of accidents.