Final answer:
The Herbst appliance is the correct device used to guide a class II jaw forward. It corrects underdeveloped or receded lower jaws, whereas other appliances serve different corrective purposes.
Step-by-step explanation:
The appliance that guides a class II jaw forward is the Herbst appliance. The Herbst appliance is an orthodontic device that facilitates the position of the lower jaw to be moved forward while the upper jaw is restrained, which assists in correcting cases where the lower jaw is underdeveloped or receded, also known as Class II malocclusions. This is different from other appliances such as the headgear which is used to correct overbites by pulling the upper jaw back, the palatal expander which widens the upper jaw to address a narrow palate, or retainers that hold teeth in place after the braces have moved them into the correct position.