The expression for entropy change given by, S = - nRln ( P_2P_1 ), holds good for

Answer: Option B

We know $$S$$ as a function of $$T$$ and $$P$$ as :
$$dS=\frac{C_{P}dT}{T}-{\left(\frac{\partial V}{\partial T}\right)}_{P}dP$$
For isothermal change $$dT=0$$   for ideal gas $$\frac{\partial V}{\partial T_P}=\frac{nR}{P}$$
$$\text{So, }dS=-nRln\left(\frac{P_2}{P_1}\right)$$
For isothermal volume change of an ideal gas for both reversible and irreversible process.