**Core Concept**
The vitreous humor is a clear gel-like substance filling the center of the eye, providing support and maintaining its shape. It is composed of water (98-99%), hyaluronic acid, and collagen fibers.

**Why the Correct Answer is Right**
During a cataract surgery, the lens is typically removed, but the vitreous humor is preserved due to its crucial role in maintaining the eye's structure and preventing retinal detachment. The vitreous humor is anchored to the retina by the vitreous base, and its removal could lead to retinal complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the aqueous humor is a clear fluid produced by the ciliary body, which is drained through the trabecular meshwork. It is not related to the vitreous humor's preservation.
**Option B:** Incorrect because the lens is typically removed during cataract surgery, and its preservation is not a priority.
**Option C:** Incorrect because the retina is a complex neural tissue that is not preserved during cataract surgery.

**Clinical Pearl / High-Yield Fact**
During cataract surgery, the surgeon must be careful not to damage the vitreous base, as it can lead to retinal detachment and vision loss.

**Correct Answer: D.**