Internal energy of an ideal gas depends only upon

 internal energy is sum of a gas of translation kinetic energy and rotational kinetic energy and vibration energy.

But as a given - for ideal gas the gas possesses only translational kinetic energy

- Since we know that - energy of a gas per mole per degree of freedom $=\frac{1}{2}\mathrm{RT}$

So, $\mathrm{U}=$ internal energy $={\mathrm{kE}}_{\mathrm{x}}+{\mathrm{kE}}_{\mathrm{y}}+{\mathrm{kE}}_{\mathrm{z}}$

$\mathrm{U}=\frac{1}{2}\mathrm{RT}+\frac{1}{2}\mathrm{RT}+\frac{1}{2}\mathrm{RT}$

$\mathrm{U}=\frac{3}{2}\mathrm{RT}\to \mathrm{U}\propto \mathrm{T}$

Hence the correct answer is (B) Temperature.