What is the term for the ability of a substance to transfer thermal energy or electric current?

The correct term for the ability of a substance to transfer thermal energy or electric current is conductivity. Conductivity refers to how well a material can conduct heat or electricity. In the case of thermal energy, materials with high thermal conductivity, such as metals, allow heat to pass through them easily, making them effective for applications that require heat transfer. For electric current, materials known as conductors, such as copper and aluminum, have high electrical conductivity, enabling them to carry electric charge with minimal resistance.

Other terms in the options relate to different properties of materials. Density refers to the mass of a substance divided by its volume, which does not directly relate to heat or electric current transfer. Viscosity describes a fluid's resistance to flow, which again is unrelated to conductivity. Ductility is the ability of a material to deform under tensile stress, commonly assessed through stretching or drawing, but it doesn’t pertain to the ability to transfer heat or current. Thus, conductivity encompasses both thermal and electrical transfer capabilities, making it the appropriate choice.