**Core Concept**
Crossbites in orthodontics refer to the malposition of teeth where the upper teeth overlap the lower teeth on one or both sides of the dental arch. Developing crossbites in children can lead to further complications if left untreated, such as speech difficulties, difficulty chewing, and orthodontic problems.

**Why the Correct Answer is Right**
The treatment of developing crossbites in children primarily involves the use of **appliance therapy**, which involves the use of removable or fixed appliances to correct the malposition of teeth. This can include the use of **Hawley retainers**, **Bionator appliances**, or **Frankel appliances**, which are specifically designed to correct crossbites by promoting proper growth and development of the jaw.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while headgear appliances can be used to treat crossbites, they are typically used in conjunction with other appliances and are not the primary treatment for developing crossbites.
**Option B:** This option is incorrect because orthodontic elastics are used to correct malocclusions after orthodontic treatment, not to treat developing crossbites.
**Option C:** This option is incorrect because surgical correction of crossbites is typically reserved for more severe cases that have not responded to appliance therapy.

**Clinical Pearl / High-Yield Fact**
It's essential for orthodontists to monitor and treat developing crossbites early to prevent further complications and ensure proper growth and development of the jaw.

**Correct Answer: D. Appliance therapy. Appliance therapy**