Which of the following best defines "thermal conductivity"?

The definition of thermal conductivity is fundamentally about how efficiently heat travels through a material. It is a physical property that quantifies the rate at which heat is conducted through a material when there is a temperature difference across it. A material with high thermal conductivity allows heat to pass through it more quickly than a material with low thermal conductivity.

This property is crucial in various applications, such as in engineering and building design, where it’s important to understand how materials will perform under thermal stress. In practical terms, if you're working with insulation materials, knowing their thermal conductivity helps you determine how well they will resist the flow of heat, which directly affects energy efficiency.

In contrast, the other options refer to related but different concepts. The total heat content of a substance refers to its internal energy and does not specifically address the rate of heat transfer. The measure of thermal energy in a system relates to the overall energy contained within the system, not how that energy is transferred. The ability of a material to retain heat describes its thermal mass or capacity, which is distinct from the material's ability to conduct heat. Therefore, the best definition of thermal conductivity is indeed the property that determines how quickly heat is transferred through a material.