## **Core Concept**
The key of occlusion, also known as the "keys to occlusion," refers to specific principles or guides used in dentistry to achieve proper occlusal relationship and function. Andrew's keys of occlusion are a set of principles used for achieving a harmonious occlusion.

## **Why the Correct Answer is Right**
Andrew's 5th key of occlusion is **the flat occlusal plane**. This principle emphasizes the importance of having a flat occlusal plane in the anteroposterior dimension to ensure even distribution of forces during occlusion and to prevent uneven wear of the teeth.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not accurately represent Andrew's 5th key of occlusion.
- **Option B:** This option is incorrect as it does not specify the flat occlusal plane.
- **Option C:** This option might relate to other principles of occlusion but does not accurately describe Andrew's 5th key.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that Andrew's keys of occlusion are crucial for dental restorations and prosthodontics, aiming to achieve a functional and stable occlusion. The flat occlusal plane helps in even distribution of masticatory forces.

## **Correct Answer:** D. flat occlusal plane.