Which of the following is a characteristic feature of plastic filling materials?

Plastic filling materials, commonly used in dental procedures, are known for their ability to harden once set. This characteristic is essential as it allows the filling to provide a durable solution for repairing cavities and restoring function to the tooth. The process of hardening typically involves polymerization, where the material undergoes a chemical reaction that transforms it from a pliable state to a rigid form, ensuring it can withstand the pressures of biting and chewing.

While other options may touch on aspects of filling materials, they do not accurately describe the core characteristic of plastic filling materials. Unlike some types of temporary materials, plastic fillings are designed for long-term use once they have hardened. Additionally, modern plastic filling materials, especially composite resins, are designed to bond with tooth structure, which helps in providing a more effective seal and improves the longevity of the filling. Lastly, issues of cost and frequency of use can vary widely based on the context, but many plastic filling materials are accessible and commonly utilized in dental practice.