**Core Concept**
Intraocular pressure (IOP) is the pressure within the eye, measured in millimeters of mercury (mmHg). Normal IOP is crucial for maintaining the health and function of the eye. It is influenced by various factors, including the production and drainage of aqueous humor.

**Why the Correct Answer is Right**
The correct range for normal IOP is between 11 and 21 mmHg. This range is essential for maintaining the structural integrity of the eye, particularly the cornea and lens. Elevated or decreased IOP can lead to various ocular disorders, such as glaucoma and retinal detachment. The aqueous humor, produced by the ciliary body, maintains IOP through a balance between production and drainage through the trabecular meshwork. A normal IOP range ensures that this balance is maintained, preventing damage to the eye's delicate structures.

**Why Each Wrong Option is Incorrect**
**Option A:** 10 - 15 mmHg is too narrow a range and does not account for individual variability in IOP. A normal IOP range must accommodate the natural fluctuations in IOP throughout the day.
**Option B:** 5 - 10 mmHg is significantly lower than the normal range, which can lead to ocular hypotension and potential damage to the eye's structures.
**Option D:** 9 - 25 mmHg is too broad and does not provide a clear indication of the normal range. While it may include the normal range, it also encompasses values that are outside the normal range.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that IOP can fluctuate throughout the day, and a single measurement may not accurately reflect the individual's normal range. Therefore, it is essential to take multiple measurements, ideally at different times of the day, to determine the patient's true normal IOP range.

**✓ Correct Answer: C. 11 - 21 mmHg**