**Core Concept:** Investment materials are substances used to create a stable, heat-resistant mold of the oral cavity, allowing for the production of high-quality partial dentures made of base-metal alloys. These materials are essential for maintaining the structural integrity and longevity of the prosthesis.

**Why the Correct Answer is Right:** The correct answer, **Option C: Zirconia**, is chosen due to its superior properties compared to the other options. Zirconia is a biocompatible ceramic material that exhibits high thermal stability, making it suitable for use in high-fusing base-metal partial dentures. It can withstand the high temperatures required during the denture production process without degradation or distortion.

**Why Each Wrong Option is Incorrect:**

A. **Option A: Autoclavable wax:** Autoclavable wax is a type of heat-resistant wax used in dental procedures, but it is not a suitable investment material for high-fusing base-metal partial dentures. Wax does not possess the necessary thermal stability to withstand the high temperatures involved in the production process.

B. **Option B: Plastic:** Plastic materials are generally not suitable for investment purposes due to their inadequate thermal stability. They cannot withstand the high temperatures required for the production process and will degrade or distort, resulting in poor-quality dentures.

D. **Option D: Glass:** Similar to plastic, glass materials are not suitable investment materials for high-fusing base-metal partial dentures. Glass lacks the necessary thermal stability, making it prone to degradation and distortion during the production process.

**Clinical Pearl:** Choosing the right investment material is crucial for the success of high-fusing base-metal partial denture production. Zirconia offers the best combination of biocompatibility, thermal stability, and resistance to thermal stress, ensuring the production of high-quality dentures with long-lasting performance.