The internal energy of one mole of a gas is given as

 The internal energy of a thermodynamic system is the total energy contained within it. It is the energy necessary to create or prepare the system in its given internal state, and includes the contributions of potential energy and internal kinetic energy. K.E. $=\frac{1}{2}{\mathrm{mv}}^2=\frac{3}{2}\mathrm{RT}$ (for 1 molecule) For $\mathrm{n}$ moles K.E. $=\frac{3}{2}\mathrm{KT}(\mathrm{k}=$ Boltzmann's constant $)$

Hence the correct answer is (D) $\frac{3}{2}\mathrm{kT}$.