(A) : Volume to temperature ratio is constant for a fixed amount of gas at constant pressure.

(R) : At constant pressure the volume of a given mass of a gas increases or decreases by 1/273 times of its volume at 0^oC, for every 1^oC change in temp.

Given data: Volume to temperature ratio is constant for a fixed amount of gas at constant pressure.

Concept used: Charle’s law

Detailed solution: From Charle’s law: Volume is proportional to temperature  for a fixed amount of gas at constant pressure.

So, assertion is correct.

T₁ = 0^o C = 273 K

⇒ $\frac{{\mathrm{V}}_2}{{\mathrm{V}}_1}=\frac{{\mathrm{T}}_2}{{\mathrm{T}}_1}$

⇒ $\frac{{\mathrm{V}}_2}{{\mathrm{V}}_1}={\mathrm{T}}_2\times \frac{1}{273}$

Thus, the Reason is correct.

Hence, option (A) is correct.