**Core Concept**
The question tests the understanding of the rotational forces acting on a distal extension base in removable partial dentures, specifically focusing on the resistance to rotation around the longitudinal axis. This concept is crucial in **prosthodontics** and **denture design**.

**Why the Correct Answer is Right**
Resistance to rotation around the longitudinal axis in a distal extension base is primarily provided by the **border seal** and **intimate tissue contact** of the denture base with the underlying mucosa. However, the most effective resistance comes from the **retentive components** and the **supporting tissues**, but in the context of rotation, **torque** is resisted by the **vertical dimension of the border** and the **peripheral seal**.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because while it may provide some level of stability, it's not the primary mechanism resisting rotation around the longitudinal axis.
**Option B:** Incorrect as it is more related to the overall fit and stability rather than specific resistance to rotational forces.
**Option C:** Incorrect because, although important for retention, it does not directly resist rotation around the longitudinal axis.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the design of the distal extension base, including its border and the use of **stress-breakers**, is critical in managing rotational forces and ensuring the long-term success of the denture.

**Correct Answer:** D. Border seal and vertical dimension of the border.