Answer:
According to Charles's law, the volume of a given amount of gas will be directly proportional to its temperature, as long as the pressure and the number of moles of the gas are held constant.
Therefore, the correct answer is C: when pressure and the number of moles remain constant.
This means that if the pressure and the number of moles of gas are kept constant, then the volume of the gas will increase as the temperature increases, and the volume will decrease as the temperature decreases. This relationship is expressed mathematically as V/T = constant, where V is the volume of the gas, T is the temperature, and the constant is a proportionality constant that depends on the specific gas and the units of measurement used.