Which type of tack would most likely be used to keep the horse's mouth closed during riding?

The flash noseband is specifically designed to help keep a horse's mouth closed while allowing some degree of flexibility. It typically consists of a regular cavesson as a base, with an additional strap that runs across the horse's lips, effectively preventing the horse from opening its mouth excessively. This can be beneficial for horses that tend to evade the bit by gaping their mouths.

In comparison, other types of nosebands may serve different functions. The kineton noseband primarily helps with control, allowing the rider to maintain a connection without too much pressure on the bit. The drop noseband works more by applying pressure to the nose, encouraging the horse to maintain a more rounded shape and can prevent the horse from leaning on the reins. The cross-over noseband also aids in control, offering pressure at different points of the horse's face but is less focused on keeping the mouth shut compared to a flash noseband.

The design and function of the flash noseband make it the most effective choice for keeping the horse's mouth closed during riding, thereby supporting better communication between horse and rider.