**Core Concept**
Optic disc size is a crucial parameter in ophthalmology, referring to the diameter of the optic disc, which is the point on the retina where the optic nerve fibers exit. The size of the optic disc can provide valuable information about the health of the optic nerve and the retina.

**Why the Correct Answer is Right**
The correct answer is 1.5 mm in diameter, which is a standard reference value for the size of the optic disc. This size is based on the average diameter of the optic disc in a normal adult eye. The optic disc size can vary slightly between individuals, but a diameter of 1.5 mm is generally considered normal. The size of the optic disc is an important factor in the diagnosis and management of various eye conditions, including glaucoma and optic neuritis.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the size of the optic disc can vary between individuals, and a diameter of 1 mm is generally considered small.
* **Option B:** This option is incorrect because the size of the optic disc can vary between individuals, and a diameter of 2 mm is generally considered large.
* **Option C:** This option is incorrect because the size of the optic disc is typically measured in millimeters, not centimeters.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that a normal optic disc size is a crucial factor in the diagnosis and management of various eye conditions. An abnormal optic disc size can be a sign of underlying disease, such as glaucoma or optic neuritis.

**Correct Answer: A. 1.5 mm**