**Core Concept**
The preparation of posterior cast crowns involves modifying the tooth structure to accommodate the crown, ensuring a proper fit and function. This includes beveling the functional cusps to enhance the crown's retention and resistance. **Dental anatomy** and **restorative dentistry** principles are crucial in this context.

**Why the Correct Answer is Right**
Beveling the functional cusps in posterior cast crowns helps in reducing the cusp angle, which decreases the stress concentration and enhances the crown's durability. This procedure also allows for a smoother transition between the crown and the opposing tooth, improving **occlusal** harmony. The **bevel** provides an additional mechanical advantage, increasing the retention of the crown.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not specifically address the functional aspect of cusp preparation.
**Option B:** This option is wrong as it doesn't relate to the preparation of posterior cast crowns.
**Option C:** This choice is incorrect because it is not relevant to the context of beveling functional cusps.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the **functional cusp bevel** is essential for the proper seating and function of posterior cast crowns, as it helps in distributing the **occlusal forces** evenly. This is crucial for the longevity and success of the restoration.

**Correct Answer:** D. Occlusally