**Core Concept**
The angle of divergence in the preparation of cast-metal intracoronal restorations refers to the optimal design of the tooth preparation to ensure proper fit and marginal seal of the cast-metal restoration. This involves creating a diverging angle between the axial wall and the marginal wall of the preparation to allow for the contraction of the metal during setting.

**Why the Correct Answer is Right**
The correct answer is related to the principle of ensuring a proper marginal seal and preventing the formation of gaps between the restoration and the tooth. When the metal contracts during setting, a diverging angle allows the metal to flow outward and form a tight seal with the tooth. This is essential for preventing bacterial leakage and ensuring the longevity of the restoration. The angle of divergence also helps to distribute the forces of mastication more evenly, reducing the risk of restoration failure.

**Why Each Wrong Option is Incorrect**
**Option A:** A converging angle would actually increase the risk of marginal gaps and bacterial leakage, as the metal would be more likely to contract inward and create a gap between the restoration and the tooth.
**Option B:** A flat angle would provide little to no benefit in terms of marginal seal or restoration longevity, as the metal would be unable to flow and form a tight seal with the tooth.
**Option C:** An acute angle would be too sharp and could increase the risk of restoration failure, as the metal would be more likely to fracture or crack under the forces of mastication.

**Clinical Pearl / High-Yield Fact**
The angle of divergence should be at least 10-15° to ensure a proper marginal seal and prevent bacterial leakage.

**Correct Answer:** 10-15°.