**Core Concept**
The axial wall of a tooth preparation for silver amalgam restoration is crucial for the success of the restoration. A well-prepared axial wall ensures the proper placement and seating of the amalgam material, thereby preventing marginal leakage and promoting a durable restoration. The axial wall should be prepared to follow the natural contours of the tooth, ensuring a proper adaptation of the restoration to the tooth structure.

**Why the Correct Answer is Right**
The correct answer, D, emphasizes the importance of preparing the axial wall in a way that it follows the buccolingual contour of the tooth. This means that the axial wall should be prepared in a way that it is parallel to the long axis of the tooth and follows the natural shape of the tooth. This approach ensures that the amalgam material is properly seated and adapted to the tooth structure, preventing marginal leakage and promoting a durable restoration. The preparation of the axial wall in this manner also helps to minimize the risk of amalgam material extruding beyond the confines of the tooth preparation, which can lead to secondary caries and other complications.

**Why Each Wrong Option is Incorrect**
**Option A:** A flat, straight plane buccolingually may not provide a proper adaptation of the amalgam material to the tooth structure, leading to marginal leakage and other complications.

**Option B:** Preparing the axial wall parallel to the long axis of the tooth without considering the buccolingual contour of the tooth may result in an amalgam restoration that is not properly adapted to the tooth structure.

**Option C:** A concave axial wall may not provide sufficient bulk of amalgam material, leading to a restoration that is prone to fracture and other complications.

**Clinical Pearl / High-Yield Fact**
When preparing the axial wall of a tooth for a silver amalgam restoration, it is essential to follow the natural contours of the tooth to ensure a proper adaptation of the restoration to the tooth structure. This approach helps to minimize the risk of marginal leakage and promotes a durable restoration.

**✓ Correct Answer: D. Parallel to the long axis of the teeth and should follow the buccolingual contour of the tooth**