Which of the following would increase the average speed the ideal gas molecules?

The average speed of ideal gas molecules depends on temperature: v_rms = sqrt(3RT/M). Increasing temperature increases speed. Option D involves heating, which raises temperature. Although maintaining constant volume and pressure simultaneously is thermodynamically impossible for an ideal gas (as heating would change either pressure or volume), the key action is heating, which increases temperature and thus average speed. Options A, B, and C either decrease temperature or leave it unchanged under typical conditions (lowering temperature decreases speed; compressing with a piston may increase temperature only if adiabatic, but not always; expanding into vacuum does not change temperature for an ideal gas). Therefore, D is the best choice.