**Core Concept**
MIP-26, also known as aquaporin-0, is a type of aquaporin water channel protein primarily expressed in the lens of the eye. It plays a crucial role in maintaining the transparency and refractive index of the lens by facilitating the transport of water and ions across the lens fibers.

**Why the Correct Answer is Right**
The correct function of MIP-26 is to regulate the amount of water in the lens, which is essential for maintaining its refractive index and transparency. This is achieved through the protein's ability to form water channels, allowing water to pass through the lens fibers while restricting the movement of larger solutes. This selective permeability is critical for maintaining the lens's optical clarity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because MIP-26 is not primarily involved in the regulation of electrolyte balance in the lens.

**Option B:** This option is incorrect because while MIP-26 does facilitate the transport of ions, its primary function is not to regulate ion concentration.

**Option C:** This option is incorrect because MIP-26 is not directly involved in the synthesis of lens proteins.

**Clinical Pearl / High-Yield Fact**
MIP-26 is a key component of the lens's refractive index, and its dysfunction has been implicated in various lens-related disorders, including cataracts.

**Correct Answer: D. Maintaining lens transparency and refractive index.**