Final answer:
Removable dental appliances are most effective during middle childhood, particularly between the ages of 10 to 13, when most of the permanent teeth are emerging and the jaw is still growing.
Step-by-step explanation:
The age at which removable appliances are most effective is typically during middle childhood, when children are between 6 to 12 years old. During this period, children's deciduous teeth are replaced by their permanent teeth. The transition begins with the loss of the eight incisors, usually between ages six and eight. This is followed by the replacement of the eight premolars, generally between ages nine and twelve, and the four canines, typically between ten and thirteen years old. With the majority of permanent teeth emerging by age 13, removable orthodontic appliances can be most effective in guiding dental development and correcting malocclusions during this window of opportunity.
Therefore, the correct answer to the question "What age are removable appliances most effective?" would be b) 10-13 years.