Answer
Choice B.
Work Step by Step
As the gas expands, its volume increases and it does positive work W on its surroundings.
Since Q = 0 for an adiabatic process, the first law of thermodynamics $\Delta U = Q-W$ shows that the internal energy must decrease.
For an ideal gas, the temperature is proportional to the gas’s internal energy so the temperature decreases.
Use the ideal gas law, PV = nRT. If the volume increases as the temperature decreases, the pressure must decrease.