**Core Concept**
The primary goal of designing a distal extension partial denture is to minimize the forces transmitted to the abutment teeth, thereby preventing damage and ensuring the longevity of the prosthesis. This is achieved by optimizing the distribution of applied loads.

**Why the Correct Answer is Right**
The correct answer is **A. Use of a rigid base with a stress-breaking connector**. A rigid base with a stress-breaking connector helps to distribute the masticatory forces evenly across the denture-bearing area, reducing the stress on the abutment teeth. The stress-breaking connector, typically a metal or fiber-reinforced resin frame, is designed to absorb and dissipate the forces, thereby limiting the load on the abutment teeth.

**Why Each Wrong Option is Incorrect**
* **Option B:** Use of a flexible base alone may not be sufficient to limit the applied loads, as it may still transmit significant forces to the abutment teeth.
* **Option C:** Adding more abutment teeth may not necessarily reduce the load on the existing abutment teeth, as the additional teeth may also require support and may not distribute the forces evenly.
* **Option D:** Using a non-rigid base without a stress-breaking connector may actually increase the load on the abutment teeth, as it may not provide adequate support and may cause the denture to flex and concentrate forces on the abutment teeth.

**Clinical Pearl / High-Yield Fact**
When designing a distal extension partial denture, it is essential to balance the need for support and stability with the need to minimize the forces transmitted to the abutment teeth. A rigid base with a stress-breaking connector is a critical component of this design, as it helps to distribute the masticatory forces evenly and prevents damage to the abutment teeth.

**Correct Answer:** A. Use of a rigid base with a stress-breaking connector.