According to the equipartition of energy, each degree of freedom carries 21RT energy per mole and total energy per mole of a gas having n degrees of freedom is 21nRT. Therefore, statement A is wrong and statement B is correct.
A monoatomic molecule only has translational degrees of freedom and does not have rotational degree of freedom. For a polyatomic model there are three rotational degrees of freedom and three translational degrees of freedom. Therefore, statement C is incorrect and statement D is correct.