**Core Concept:**
Intraocular pressure (IOP) is the pressure within the eye and is an important indicator of eye health, as elevated IOP is a risk factor for glaucoma. The normal range of IOP is crucial for identifying potential complications.

**Why the Correct Answer is Right:**
The correct answer, **D**, indicates the normal range of intraocular pressure as 10-21 mmHg. This is based on the clinical assessment and measurements taken during routine eye examinations. The IOP is influenced by factors such as aqueous humor production, outflow facility, and episcleral venous pressure. Maintaining a normal IOP helps prevent damage to the optic nerve and visual pathway.

**Why Each Wrong Option is Incorrect:**

A. **15 mmHg**: This answer is incorrect because the normal IOP range is broader than 15 mmHg, spanning from 10 to 21 mmHg.

B. **25 mmHg**: This answer is incorrect as a normal eye should not have IOP values above 25 mmHg. An elevated IOP can lead to optic nerve damage and vision loss.

C. **1 mmHg**: This answer is not relevant as mmHg (millimeters of mercury) is the unit used to measure IOP, not a specific value.

**Clinical Pearl / High-Yield Fact:**
Intraocular pressure is crucial for eye health, and a normal range of IOP helps prevent damage to the optic nerve and visual pathway. Regular eye examinations can help detect any abnormalities in IOP, enabling early intervention for potential glaucoma and preserving vision.