Which of the following control mechanisms is placed on a horse's nose to act as a distraction?

The correct answer is the twitch. A twitch is a tool designed to distract a horse by applying pressure to sensitive areas, such as the skin on the nose. When used correctly, the twitch can help calm the horse and make it easier to handle during various procedures, such as veterinary examinations or farrier work. The distraction caused by the twitch helps to divert the horse's attention away from any discomfort or anxiety, allowing for a smoother process.

In contrast, the chin strap, side reins, and browband serve different purposes. A chin strap primarily supports the bridle and helps keep it in place, while side reins are used in training to encourage submission and proper head carriage. The browband, which is found on the bridle, simply keeps the bridle positioned correctly on the horse's head. None of these options serve the specific function of distraction in the same way that a twitch does.