**Core Concept**
The concept of cusp protection is crucial in restorative dentistry, particularly in cast restorations, to maintain the integrity and longevity of the tooth. This involves preserving the cusp tip and the marginal ridge to ensure proper occlusal function and prevent further tooth wear.

**Why the Correct Answer is Right**
Cusp protection is essential when a cast restoration is performed, as it helps to prevent the destruction of the cusp tip and marginal ridge. This is achieved by preserving the occlusal morphology of the tooth and ensuring that the restoration does not compromise the cusps. The marginal ridge plays a crucial role in supporting the cusps and maintaining the occlusal plane, and its destruction can lead to tooth wear and further restorative needs.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because cusp protection is not limited to specific types of restorations, but is a general principle in restorative dentistry.

**Option B:** This option is incorrect because cusp protection is not only necessary for posterior teeth, but also for anterior teeth, particularly those with significant occlusal forces.

**Option C:** This option is incorrect because while cusp protection is important, it is not the only factor in determining the success of a cast restoration. Other factors, such as material selection and proper fitting, also play a crucial role.

**Clinical Pearl / High-Yield Fact**
A key principle in cusp protection is to preserve the marginal ridge and cusp tip, as these areas are critical in maintaining the occlusal function and preventing further tooth wear. This can be achieved by using a conservative approach to tooth preparation and ensuring that the restoration does not compromise these areas.

**Correct Answer: D.**