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:
[tex] \frac{V}{T}=k [/tex]
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.