**Core Concept**
Angular bone defects refer to the loss of bone structure at the angle of the mandible, typically resulting from trauma, infection, or surgical intervention. These defects can significantly impact the functional and aesthetic outcomes of dental rehabilitation. The classification of angular bone defects is crucial for determining the best treatment approach.

**Why the Correct Answer is Right**
The correct classification system for angular bone defects was proposed by Bell. It categorizes these defects based on their width and depth. The classification is as follows:
- Type I: Vertical bone loss less than 10 mm
- Type II: Vertical bone loss 10-20 mm
- Type III: Vertical bone loss greater than 20 mm

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not refer to a recognized classification system for angular bone defects.
**Option B:** This option is incorrect as it does not accurately describe the Bell classification system.
**Option C:** This option is incorrect as it is not a recognized classification system for angular bone defects.

**Clinical Pearl / High-Yield Fact**
When classifying angular bone defects, it is essential to consider the width and depth of the defect to determine the best treatment approach. The Bell classification system provides a standardized framework for evaluating and treating these defects.

**Correct Answer: D. Bell classification system**