**Core Concept**
Tooth preparation involves removing enamel and dentin to create a cavity for a dental restoration. The goal of tooth preparation is to remove the minimum amount of tooth structure necessary to achieve an ideal restoration while preserving tooth integrity and preventing further decay.

**Why the Correct Answer is Right**
The minimum amount of taper that should be maintained for an ideal tooth preparation is 10 degrees. This is because a 10-degree taper allows for easy placement and removal of the restoration, while also preventing the restoration from being too loose or too tight. A taper of 10 degrees also helps to distribute the forces of mastication (chewing) evenly across the restoration, reducing the risk of fracture. This is achieved by maintaining a minimum of 1 mm of axial wall thickness.

**Why Each Wrong Option is Incorrect**
* **Option A:** A taper of 5 degrees may not provide sufficient stability for the restoration, increasing the risk of fracture or microleakage.
* **Option B:** A taper of 15 degrees may be too aggressive and result in an overly thin axial wall, increasing the risk of restoration failure.
* **Option C:** A taper of 20 degrees is excessive and may lead to a restoration that is too loose, increasing the risk of leakage and secondary caries.
* **Option D:** This option is blank and does not provide a taper value.

**Clinical Pearl / High-Yield Fact**
When preparing a tooth for a restoration, always maintain a minimum of 1 mm of axial wall thickness and a taper of 10 degrees to ensure a stable and long-lasting restoration.

**Correct Answer: . 1.5**