**Core Concept**
The hollow ground bevel is a type of dental instrument edge preparation, specifically designed for precise cutting and minimal damage to the tooth structure. It involves grinding the bevel to create a concave surface, which enhances the instrument's ability to remove small amounts of tooth material without chipping or fracturing.

**Why the Correct Answer is Right**
The hollow ground bevel is created by grinding the bevel at an angle, typically between 20° to 30°, to produce a concave surface. This design allows for more efficient removal of tooth material and reduces the risk of tooth fracture. The concave surface also helps to minimize the amount of tooth structure that is removed, making it an ideal choice for precision cutting.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a hollow ground bevel is not created by simply rounding the bevel. Rounding the bevel would not produce the concave surface necessary for precision cutting.
**Option B:** This option is incorrect because a hollow ground bevel is not created by sharpening the bevel at a 90° angle. Sharpening at a 90° angle would not produce the concave surface necessary for precision cutting.
**Option C:** This option is incorrect because a hollow ground bevel is not created by grinding the bevel to a flat surface. Grinding to a flat surface would not produce the concave surface necessary for precision cutting.

**Clinical Pearl / High-Yield Fact**
To create a hollow ground bevel, dental instruments should be sharpened at an angle between 20° to 30°, and the bevel should be ground to produce a concave surface. This technique is essential for precision cutting and minimizing tooth damage.

**Correct Answer: C. Flat surface.**