The correct answer is Biocompatibility.

In the context of dental materials, properties can be broadly categorized into biological, physical, and mechanical properties.

- Biological properties relate to how the material interacts with the biological tissues in the oral environment. This includes the material's ability to perform without causing adverse reactions such as toxicity, allergy, or irritation. Therefore, biocompatibility is the key biological property that ensures the material is safe and compatible with the body’s tissues.
- Thermal conductivity and solubility are examples of physical properties, as they describe how the material behaves physically under temperature change or in the presence of fluids.
- Compressive strength is a mechanical property that measures the material's ability to withstand forces without failure.

In summary, biocompatibility defines the biological property of dental materials, ensuring safe integration with oral tissues—a critical factor in the long-term success of dental restorations.

Reference: Craig’s Restorative Dental Materials, 14th Edition, Chapter 2: Biological Properties of Dental Materials / Page 25