**Core Concept**
A three-quarter crown preparation on a mandibular molar requires structural durability to withstand occlusal forces. The preparation involves removing significant tooth structure, necessitating the incorporation of specific design features to enhance its resistance to fracture and wear.

**Why the Correct Answer is Right**
The occlusal shoulder, functional cusp bevel, and proximal groove all contribute to the structural durability of a three-quarter crown preparation. The **occlusal shoulder** acts as a stress-breaker, dissipating occlusal forces and reducing the risk of fracture. The **functional cusp bevel** helps to distribute stresses more evenly, minimizing the risk of cusp fracture. The **proximal groove** provides additional support to the tooth and helps to prevent proximal overhangs that could compromise the preparation's integrity.

**Why Each Wrong Option is Incorrect**
* **Option A:** An occlusal shoulder alone is insufficient for imparting structural durability, as it only addresses occlusal forces and neglects other potential stressors.
* **Option B:** A functional cusp bevel is essential, but it is not the sole contributor to structural durability; other features are also necessary.
* **Option C:** A proximal groove is crucial, but like the functional cusp bevel, it is not the only factor; all features are necessary for optimal durability.

**Clinical Pearl / High-Yield Fact**
When designing a three-quarter crown preparation, it is essential to consider the interplay between occlusal forces, cusp morphology, and proximal tooth structure. This requires a comprehensive understanding of the biomechanics involved, as well as the ability to balance competing demands for durability, aesthetics, and function.

✓ Correct Answer: D. All of the above