**Core Concept**
Agar is a polysaccharide extracted from red algae, commonly used as a gelling agent in microbiological media. It provides a solid surface for microbial growth, allowing for the isolation and cultivation of microorganisms.

**Why the Correct Answer is Right**
Agar is added to broth medium to solidify it, creating a semisolid or agar plate. This solidification enables the growth of microorganisms in a controlled environment, facilitating the observation of colony morphology, size, shape, color, and other characteristics. Agar also prevents the sedimentation of bacterial cells, allowing for easier counting and identification.

**Why Each Wrong Option is Incorrect**
**Option A:** Agar is not added to broth medium for its antibacterial properties. While agar does have some antimicrobial effects, this is not its primary purpose in microbiological media.
**Option B:** Agar is not added to broth medium to increase its viscosity. While agar does increase the viscosity of the medium, this is a secondary effect of its primary purpose as a gelling agent.
**Option C:** Agar is not added to broth medium to support the growth of specific microorganisms. While agar can support the growth of a wide range of microorganisms, it is not added specifically to promote the growth of certain species.

**Clinical Pearl / High-Yield Fact**
When preparing agar plates, it is essential to ensure that the agar is fully dissolved and the medium is sterile to prevent contamination and promote the growth of the desired microorganism.

**Correct Answer: C. This is the primary purpose of adding agar to broth medium.**