**Core Concept**
The macula lutea is a specialized region in the retina responsible for central vision and fine detail. Its diameter is an important anatomical measurement that helps in understanding various retinal diseases and conditions.

**Why the Correct Answer is Right**
The correct answer is related to the size of the macula lutea, which is approximately 5-6 mm in diameter. This measurement is crucial for diagnosing and managing conditions like macular degeneration, where the macula's function is compromised. The macula's size is also important for understanding other retinal diseases, such as macular edema or macular dystrophies.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the diameter of the macula lutea is not typically measured in millimeters of arc.
**Option B:** Incorrect because the diameter of the macula lutea is not usually measured in terms of the optic disc's diameter.
**Option C:** Incorrect because the diameter of the macula lutea is not typically measured in terms of the retinal vein's diameter.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the macula lutea's size and function are critical for central vision, and any abnormalities in its size or function can lead to significant visual impairment.

**Correct Answer: C. 5-6 mm.**