How many rotational degrees of freedom does a rigid diatomic molecule have?

The number of translational degrees of freedom for a diatomic gas is

The total kinetic energy of $1$ mole of oxygen at $27^{\circ} C$ is:
[Use universal gas constant $(R) = 8.31 \ J/mol \cdot K$] (in $J$)

The ratio of specific heats of a gas is $\frac{9}{7}$. The number of degrees of freedom of the gas molecules for translational motion is:

If the internal energy of $3$ moles of a gas at a temperature of $27^{\circ} C$ is $2250 R$,then the number of degrees of freedom of the gas is (where $R$ is the universal gas constant).

State and explain the law of equipartition of energy.