**Core Concept**
Overdentures are a type of removable dental prosthesis that is used to replace missing teeth. They are designed to fit over a limited number of remaining natural teeth or dental implants, providing support and stability to the prosthetic teeth.

**Why the Correct Answer is Right**
Overdentures are typically used to preserve the alveolar bone and surrounding tissues, preventing further resorption and atrophy. By distributing the forces of mastication across the remaining natural teeth or implants, overdentures help to maintain the overall health and function of the oral cavity. The use of overdentures also allows for better retention and stability of the prosthesis, reducing the risk of denture-related complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Overdentures are not typically used for aesthetic purposes, as they are designed to provide functional support and stability rather than cosmetic appeal.
**Option B:** While overdentures can be used in conjunction with dental implants, they are not exclusively reserved for implant-supported restorations.
**Option C:** Overdentures are not a type of fixed dental prosthesis, as they are removable and designed to be taken out and cleaned regularly.

**Clinical Pearl / High-Yield Fact**
A key consideration when using overdentures is the importance of maintaining good oral hygiene and regular follow-up appointments with a dentist to ensure proper fit and function.

**Correct Answer: D.**