**Core Concept**
The question is testing the understanding of the properties of agar hydrocolloid impression material, a type of dental impression material used in dentistry. Agar hydrocolloid impression material is a hydrocolloid gel that undergoes syneresis, a process where it releases water and becomes stronger.

**Why the Correct Answer is Right**
The correct answer is **C. Borax**. Borax (sodium tetraborate) is a salt that increases the strength and reduces the viscosity of agar hydrocolloid impression material by decreasing its water content. When borax is added to the agar mixture, it reacts with the water to form a more stable gel network, resulting in increased strength and reduced viscosity. This allows the impression material to be more rigid and less prone to deformation, making it suitable for detailed impressions.

**Why Each Wrong Option is Incorrect**
* **Option A:** Glycols do not have a significant effect on the properties of agar hydrocolloid impression material. They may be used as disinfectants or as a component of impression trays, but they do not alter the material's physical properties.
* **Option B:** Glycerin is a humectant that can be used to prevent the drying out of agar hydrocolloid impression material. However, it does not increase the strength or reduce the viscosity of the material.
* **Option D:** None of the other options provided are correct.

**Clinical Pearl / High-Yield Fact**
When working with agar hydrocolloid impression material, it's essential to use the correct amount of borax to achieve the desired consistency. Too little borax may result in a weak impression, while too much borax can cause the material to become too rigid and difficult to handle.

**Correct Answer:** C. Borax.