At very low temperatures, the heat capacity $$\text{C}_\text{V}$$​ of a solid is:

Correct option is A

​$$C_V = \left( \dfrac{12 \pi^4}{5} \right) N k_B \left( \dfrac{T}{\theta} \right)^3\\\text{Where:}\\\begin{aligned}N &\text{ is the number of atoms,} \\k_B &\text{ is the Boltzmann constant,} \\T &\text{ is the temperature,} \\\theta &\text{ is the Debye temperature.}\end{aligned}$$​​