What is the free energy change, when one mole of water at $100°\mathrm{C}$ and $1\mathrm{ }\mathrm{a}\mathrm{t}\mathrm{m}$ pressure is converted into steam at $100°\mathrm{C}$ and $1\mathrm{ }\mathrm{a}\mathrm{t}\mathrm{m}$ pressure?

$\mathrm{\Delta G}=\mathrm{RT} \ln \frac{{\mathrm{P}}_2}{{\mathrm{P}}_1}$

Since, here ${\mathrm{P}}_2={\mathrm{P}}_1$

$\mathrm{\Delta G}=0$

Thus, the free energy change, when one mole of water at $100°\mathrm{C}$ and $1\mathrm{ }\mathrm{a}\mathrm{t}\mathrm{m}$ pressure is converted into steam at $100°\mathrm{C}$ and $1\mathrm{ }\mathrm{a}\mathrm{t}\mathrm{m}$ pressure is zero calorie.

Hence, option (D) is correct.