Four cylinders hold equal numbers of moles of argon, hydrogen,...

The internal energy of an ideal gas is given by $U = \dfrac{f}{2} nRT$, where $f$ is the number of degrees of freedom.

Since the number of moles $n$ and temperature $T$ are the same for all four gases, the internal energy is directly proportional to the degrees of freedom $f$.

For argon (monoatomic gas), $f = 3$.

For hydrogen and nitrogen (diatomic gases), $f = 5$.

For carbon dioxide (polyatomic gas), $f \ge 5$.

Since argon has the minimum number of degrees of freedom, its internal energy is the minimum.