**Core Concept**
The distal extension removable partial denture (RPD) requires a part that provides stability and support to the abutment teeth, particularly against movement and torquing forces. This part is essential for the overall success of the RPD.

**Why the Correct Answer is Right**
The part that provides the greatest resistance to movement and torquing of the abutment teeth in a distal extension RPD is the **clasp assembly**. The clasp assembly consists of a clasp arm, a clasp tip, and a retentive arm. The clasp arm is the part of the clasp that wraps around the abutment tooth and provides resistance to movement and torquing forces. The clasp arm is typically made of a thin, flexible material that allows it to engage with the undercut area of the abutment tooth. This engagement provides the necessary resistance to movement and torquing forces, ensuring the stability of the RPD.

**Why Each Wrong Option is Incorrect**
* **Option A:** The **distal extension base** provides support and stability to the RPD but does not provide the greatest resistance to movement and torquing of the abutment teeth.
* **Option B:** The **retentive arm** is part of the clasp assembly but is not responsible for providing the greatest resistance to movement and torquing of the abutment teeth.
* **Option D:** The **precision attachment** is a type of retentive mechanism used in RPDs but is not the part that provides the greatest resistance to movement and torquing of the abutment teeth.

**Clinical Pearl / High-Yield Fact**
When designing a clasp assembly for a distal extension RPD, it is essential to consider the type of clasp and the material used to ensure optimal resistance to movement and torquing forces. A well-designed clasp assembly can significantly improve the stability and success of the RPD.

**Correct Answer:** C. Clasp assembly.