ANS:A - reversible

When a gas in a vessel expands and its internal energy decreases, the process involved is irreversible. In an irreversible process, the system undergoes changes that cannot be exactly reversed to restore the system to its original state. In the case of gas expansion where internal energy decreases, it typically occurs due to the gas doing work on its surroundings, such as pushing a piston. During this expansion, the gas loses internal energy as it performs work, resulting in a decrease in its internal energy. Conversely, in a reversible process, the system undergoes changes in such a way that it can be exactly reversed by an infinitesimal change in a property. Isothermal and adiabatic processes can be reversible or irreversible depending on the conditions and constraints applied to the system. However, in this case, since the process results in a decrease in internal energy, it indicates irreversibility.