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.