Final answer:
Applying heat to a gas in a piston chamber causes the gas particles to speed up, increasing their kinetic energy and volume to maintain constant pressure.
Step-by-step explanation:
When heat is applied to a gas in a piston chamber and the piston is allowed to move to keep the pressure constant, the particles of the gas gain kinetic energy. As the gas particles speed up, they collide more frequently and with greater force against the piston. This action causes the piston to move outward, increasing the volume of the gas to maintain constant pressure, in accordance with Charles's Law. The increase in the average kinetic energy of the gas particles makes the gas hotter. Hence, the correct answer is that the gas particles will speed up (A).