For an ideal gas, the internal energy depends upon its __________ only.

Actually the internal energy ($$U$$) of a substance is a function of $$dU = CvdT - \left[ {P + T\left( {\frac{{\left( {\frac{{\partial V}}{{\partial T}}} \right)P}}{{\left( {\frac{{\partial V}}{{\partial P}}} \right)T}}} \right)dV} \right]$$

For an ideal gas, $$PV = RT$$

So, $$\left( {\frac{{\partial V}}{{\partial T}}} \right)P = \frac{R}{P}$$   and  $$\left( {\frac{{\partial V}}{{\partial P}}} \right)T = - \frac{{RT}}{{{P^2}}}$$
Hence, $$dU = CvdT$$
So, Internal energy of an ideal gas is purely a function of temperature. (Here the $${C_V}$$ is considered only as a function of temperature which is satisfied by many of the substances.)