The expression, G = nRT.l n P_2 P_1, gives the free energy change

Answer: Option A

We know, the property relation:
$$dG=vdp-sdT$$
Which is valid for both reversible and irreversible process since it is a property relation. when the system undergoes isothermal change $$dT=0$$
$$\begin{array}{rl}& \text{So, }dG=vd\\ & \Rightarrow dG=\frac{nRT}{P}dP\left(\text{for, ideal gas}\right)\end{array}$$
On, integration
$$\Rightarrow \Delta G=nRT.ln\frac{P_2}{P_1}$$
So, for a system containing ideal gas and undergoing isothermal change of volume or pressure this expression is valid.