ANS:B - heat capacities of solids.

Kopp's rule is a principle in solid-state physics and chemistry that relates the molar heat capacity of a solid compound to the molar heat capacities of its constituent elements. Specifically, Kopp's rule states that the molar heat capacity of a compound is approximately equal to the sum of the molar heat capacities of its constituent elements, each multiplied by a certain numerical factor. Therefore, Kopp's rule is useful for the determination of the heat capacities of solids. So, the correct answer is: heat capacities of solids.