Final answer:
To guard against a horse bolting when removing a halter, a handler should use proper techniques and maintain control by standing close and keeping a firm grasp on the lead rope.
Step-by-step explanation:
When removing a halter from a horse, a handler can best guard against the horse bolting by using proper techniques and being mindful of the horse's behavior. First, the handler should stand close to the horse's shoulder to maintain control. Then, the handler should use one hand to undo the halter buckle while keeping a firm grip on the lead rope with the other hand. This way, if the horse tries to bolt, the handler can quickly regain control by pulling on the lead rope.