**Core Concept**
In keratoplasty, also known as corneal transplant, the goal is to replace the diseased or damaged cornea with a healthy donor cornea. The size of the donor cornea is crucial to ensure a proper fit and minimize complications. The standard size of the donor cornea is typically 7-8 mm in diameter.

**Why the Correct Answer is Right**
The correct answer is based on the standardization of corneal grafts to minimize postoperative complications such as graft rejection, chronic inflammation, and vision loss. A donor cornea that is too large can lead to complications like iris incarceration, while a graft that is too small may lead to chronic inflammation and vision problems. The 7-8 mm diameter is considered optimal as it provides a good balance between the size of the recipient's cornea and the donor graft.

**Why Each Wrong Option is Incorrect**

* **Option A:** 5-6 mm - This size is too small for a standard keratoplasty and may lead to chronic inflammation and vision problems.
* **Option C:** 9-10 mm - This size is too large and may increase the risk of complications like iris incarceration and chronic inflammation.
* **Option D:** 10-12 mm - This size is excessively large and may lead to severe complications, including vision loss and chronic inflammation.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the size of the donor cornea may vary depending on the specific indication for keratoplasty, such as Fuchs' endothelial dystrophy or keratoconus. However, the standard size of 7-8 mm in diameter is widely accepted as the optimal size for most corneal transplants.

**Correct Answer: C. 9-10 mm**