As per Charle's law we can say that if pressure remains constant then
by ideal gas equation we have
[tex]PV = nRT[/tex]
[tex]V = \frac{nR}{P} T[/tex]
here
[tex]\frac{nR}{P} = constant[/tex]
so
[tex]V = CT[/tex]
here
[tex]\frac{V_1}{V_2} = \frac{T_1}{T_2}[/tex]
So here temperature must be in SI units
So the unit of temperature must be KELVIN (K)