**Core Concept**
Cast restorations require a certain degree of taper in the cavity wall to ensure proper seating and retention of the restoration. The ideal taper is crucial for maintaining the structural integrity of the tooth and preventing the restoration from becoming dislodged over time.

**Why the Correct Answer is Right**
The correct answer is C. 2-5 degree because a taper of 2-5 degrees allows for adequate retention of the cast restoration while minimizing the risk of tooth fracture. A taper that is too great (e.g., 30-45 degrees) would compromise the structural integrity of the tooth, whereas a taper that is too small (e.g., none of the above) would make it difficult to achieve adequate retention of the restoration. The optimal taper is achieved through careful planning and preparation of the cavity wall, taking into account the size and shape of the restoration.

**Why Each Wrong Option is Incorrect**
**Option A:** A taper of 30-45 degrees is too great and would increase the risk of tooth fracture, making it an unsuitable option for cast restorations.

**Option B:** A taper of 5-10 degrees is slightly greater than the optimal range and may compromise the retention of the restoration, making it a less desirable option.

**Clinical Pearl / High-Yield Fact**
For cast restorations, a taper of 2-5 degrees is generally considered optimal, as it balances the need for retention with the need to minimize the risk of tooth fracture.

**Correct Answer Line**
✓ Correct Answer: C. 2-5 degree