**Core Concept**
The sclera is the white, tough, outer layer of the eyeball, providing protection and structure to the eye. The sclera is composed of collagen and elastin fibers, and its thickness varies across different regions of the eye.

**Why the Correct Answer is Right**
The thinnest portion of the sclera is located near the limbus, which is the area where the sclera meets the cornea. The limbal region is thinner due to the presence of the ciliary body and the root of the iris, which are essential structures for lens accommodation and pupil constriction. The thinning of the sclera at this region allows for greater flexibility and mobility of the eye.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because the scleral thickness varies across different regions of the eye, but the thinnest portion is not located at the posterior pole.
* **Option B:** This is incorrect because the scleral thickness does not correlate with the presence of the optic nerve.

**Clinical Pearl / High-Yield Fact**
The sclera is the thinnest at the limbal region due to the presence of the ciliary body and the root of the iris, which are essential for lens accommodation and pupil constriction.

**Correct Answer:** C.