**Core Concept**
The question pertains to the classification of elastomers used in dentistry, which are crucial for fabricating dental appliances. Elastomers are classified based on their physical properties, such as rigidity, elasticity, and durability. The question specifically asks about the most rigid elastomer.

**Why the Correct Answer is Right**
The most rigid elastomer is Silastic (silicone-based). It is a type of addition-cure silicone elastomer, which is widely used in dentistry for making dental impressions, border sealants, and other appliances. Silastic has a high modulus of elasticity, making it the most rigid among the various types of elastomers. Its rigidity is due to the strong covalent bonds formed between the silicone molecules during the curing process.

**Why Each Wrong Option is Incorrect**
**Option A:** Polyether elastomers are less rigid compared to Silastic due to their lower modulus of elasticity. They are more flexible and commonly used for making dental impressions.
**Option B:** Condensation-cure silicone elastomers are softer and more flexible than Silastic. They are used for making dental border sealants and other appliances that require flexibility.
**Option C:** Polyvinyl siloxane (PVS) elastomers are highly elastic and flexible, making them unsuitable for applications requiring rigidity.

**Clinical Pearl / High-Yield Fact**
Silastic is a popular choice for making dental appliances due to its excellent dimensional stability, chemical resistance, and biocompatibility.

**Correct Answer:** C. Polyether elastomers are less rigid compared to Silastic due to their lower modulus of elasticity. They are more flexible and commonly used for making dental impressions.