Which bit type is similar to the Pelham but is primarily used in dressage?

The bridoon is the correct answer as it is used in conjunction with a double bridle, which is common in dressage. Similar to the Pelham bit, a bridoon features a slight curb action when used together with a Weymouth bit, allowing for both direct reins and leverage, which can provide more nuanced communication with the horse in dressage movements.

In contrast, the other options do not serve the same role in dressage. The hackamore operates without a bit and uses leverage similar to a curb, but it is not traditional for dressage. The single joint snaffle, while commonly used in various disciplines, does not provide the same level of control and adjustment as a bridoon in combination with a Weymouth. Lastly, a western curb bit is designed primarily for Western riding disciplines, which differ significantly from the needs and styles of dressage, making it inappropriate for this context.