Which of the following is a disqualification in bareback bronc riding?

In bareback bronc riding, the disqualification for failing to mark out is critical because it refers to a specific rule that requires riders to have their boots above the horse's shoulders at the start of the ride. This rule is in place to ensure that the rider is positioned correctly, demonstrating proper technique and control when the horse begins its bucking. Not adhering to this requirement results in an automatic disqualification, as it is considered a violation of the sport's standards for safety and fair play.

The other choices do not result in disqualification under standard competition rules. Staying on for 10 seconds would typically mean the rider successfully completes the ride, and using one hand is actually a requirement in bareback riding since riders are allowed to use one hand to hold onto the rigging while the other must remain free. Also, while wearing unsharpened rowels may be advised for safety reasons, it does not constitute a disqualification in professional riding events if the other rules are followed correctly.