**Core Concept**
The question is testing the modification of materials used in dentistry, specifically vinyl polysiloxane (VPS), a type of impression material. The underlying principle is the alteration of its physical properties to make it more suitable for clinical use.

**Why the Correct Answer is Right**
To make VPS hydrophilic, a hydrophilic monomer is added to the mixture. This modification allows the material to absorb more water, making it more compatible with the oral environment. The hydrophilic monomer increases the wettability of the material, which is essential for creating accurate impressions of the oral cavity. This property also helps in preventing the material from shrinking during the setting process.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify a hydrophilic monomer, and its addition would not necessarily make VPS hydrophilic.
* **Option B:** This option is incorrect because it does not relate to the modification of VPS, and its addition would not have a significant effect on the material's hydrophilicity.
* **Option C:** This option is incorrect because it is a type of additive used in VPS, but it does not necessarily make the material hydrophilic.

**Clinical Pearl / High-Yield Fact**
To remember the modification of VPS, recall that adding a hydrophilic monomer increases the material's wettability and compatibility with the oral environment. This modification is essential for creating accurate impressions and preventing material shrinkage during the setting process.

**Correct Answer: C. Hydrophilic monomer.**