According to charles' law, the volume of a fixed amount of gas is directly proportional to

The correct answer is "absolute temperature".

In fact, Charle's law states that, for a fixed amount of gas kept at constant pressure, the ratio between the volume of the gas V and its absolute temperature T is constant:


And this equation can be also read as: when V changes, T changes by the same proportion, so this means that the volume V is directly proportional to the absolute temperature T of the gas.