When a gas in a vessel expands, its internal energy decreases. The process involved is

When a gas expands in a vessel, its molecules do work on the surroundings by pushing against external pressure.

Internal energy of a gas is directly related to its temperature. If the internal energy decreases during expansion, it means the gas is losing energy.

If the system is not allowed to exchange heat with the surroundings (Q = 0), the energy decrease can only be due to the work done by the gas. This condition defines an adiabatic process.

In contrast, an isothermal process maintains constant internal energy, and reversible/irreversible refers to the manner of expansion, not the energy change.

Hence, the correct answer is Adiabatic.