**Core Concept**
The equatorial diameter of the lens is a critical anatomical parameter in ophthalmology, measuring the widest part of the lens from one equatorial edge to the other. This dimension is essential for understanding the lens's shape, size, and position within the eye. The equatorial diameter is particularly relevant in the context of lens diseases, trauma, and surgical procedures.

**Why the Correct Answer is Right**
The correct answer, 9mm, reflects the average equatorial diameter of a normal adult human lens. The lens's equatorial diameter is approximately 9mm, which allows it to maintain its natural curvature and facilitate proper accommodation. The lens's shape and size are crucial for its function in focusing light on the retina. The equatorial diameter is also an important consideration in cataract surgery, where the lens is removed and replaced with an intraocular lens implant.

**Why Each Wrong Option is Incorrect**
**Option A:** 7mm is too small, which would result in a lens that is too narrow and potentially unable to accommodate properly.
**Option B:** 8mm is close but not accurate, as it would still result in a lens that is slightly too narrow for optimal accommodation.
**Option D:** 10mm is too large, which would result in a lens that is too wide and potentially cause visual disturbances or other complications.

**Clinical Pearl / High-Yield Fact**
In ophthalmic surgery, the equatorial diameter of the lens is a critical factor in determining the size and type of intraocular lens implant to be used. A lens that is too small or too large can lead to postoperative complications, such as visual disturbances or cataract reformation.

**✓ Correct Answer: C. 9mm**