Law of equipartition of energy is used to:

According to the law of energy, total energy of a system (solid, liquid or gas) is equally distributed among its various degrees of freedom.

The Law of Equipartition of Energy is a fundamental principle in thermodynamics that states that energy is equally distributed among all available degrees of freedom in a system at thermal equilibrium. This means that each degree of freedom (whether translational, rotational, or vibrational) contributes the same amount of energy. The law is crucial in predicting the specific heats of gases and solids.

In the context of gases, the equipartition theory helps in predicting the specific heat by considering the contributions of each type of molecular motion (translation, rotation, and vibration). For an ideal gas, each degree of freedom contributes a specific amount of energy, which can be used to calculate the molar specific heat.

When applied to solids, the law is used to estimate the specific heat by analyzing how energy is distributed among the atoms or ions in the solid lattice structure. As the atoms vibrate, each vibration mode contributes energy to the system, and the law of equipartition helps in understanding how energy is partitioned among these vibrational modes.

Thus, the correct answer is C, as the law is applicable to both gases and solids, providing insights into their specific heats.