Which of the following actions is specifically prohibited while riding the slide?

Kneeling while sliding is specifically prohibited because it increases the risk of injury. When a rider is in a kneeling position, they may be more susceptible to losing balance and the slide's dynamic forces could lead to a dangerous position or impact with the slide's structure. This position can also create a risk of tipping over, which could result in accidents for both the rider and other patrons using the slide.

Riding with arms crossed, sliding feet first, and riding with a friend may have specific rules and conditions under which they are allowed, but kneeling does not provide the necessary stability and positioning that is safe for riding down the slide. Safety protocols are put in place to ensure that all participants have a fun experience while minimizing the chances of falls or injuries.