Answer
Option $
\text { (c) } \Delta s=R \ln \left(P_1 / P_2\right)
$
Work Step by Step
For a reversible isothermal process involving an ideal gas, the entropy change of the gas is given by: $$\Delta 𝑠=\frac{q}{T}.$$ But since it's an ideal gas undergoing isothermal expansion or compression, we can also express the entropy change in terms of pressure: $$\Delta s=R \ln \left(\frac{P_1}{P_2}\right).$$ Option $
\text { (c) } \Delta s=R \ln \left(P_1 / P_2\right)
$ is correct.
