Final answer:
In an adiabatic reversible expansion of an ideal gas, the internal energy does decrease, contrary to the student's implied statement that it does not. This is due to the work done by the expanding gas, as no heat is exchanged with the environment in an adiabatic process, the correct option is 1).
Step-by-step explanation:
During the adiabatic reversible expansion of an ideal gas, it is not true that the internal energy of the gas decreases. In an adiabatic process, the system is insulated so that no heat is transferred to or from the surrounding environment.
Due to this insulation, when the gas expands it does work on its surroundings and therefore its internal energy must decrease to conserve energy.
Consequently, we observe a decrease in the temperature of the gas (temperature of the gas decreases) and a decrease in pressure (pressure of the gas decreases) due to the expansion, while the volume of the gas increases (volume of the gas increases).
Therefore, all the other options apart from the internal energy decreasing are true for an adiabatic reversible expansion of an ideal gas.