What type of dental impression material is irreversible and hydrocolloid?

The correct answer is based on the properties of dental impression materials. Alginate is a type of irreversible hydrocolloid, which means that once it sets, it cannot return to its original state. This material is derived from seaweed and is widely used in dentistry for making impressions due to its ease of use, cost-effectiveness, and ability to capture fine details of the oral structures.

Alginate impressions are often taken for diagnostic purposes, making study models, or for use in custom trays. The irreversible aspect is significant because it highlights that once the alginate has set, it must be removed and cannot be re-wetted or re-used, which is a crucial characteristic when working with dental impressions.

In contrast, other materials listed don't fit this description. Polyvinyl siloxane and silicone are known for their reversible properties and can provide more precise details, while wax is primarily used for bite registration and does not fall under the hydrocolloid category. Thus, alginate stands out as the appropriate choice for an irreversible hydrocolloid impression material.