Step 1: Thermodynamics Formula
Use the Gibbs free energy equation: $\Delta G = \Delta H - T\Delta S$.
Step 2: Equilibrium Condition
At 373 K and 1 atm, water and steam are in equilibrium. For any system at equilibrium, $\Delta G = 0$.
Step 3: Result
Substituting $\Delta G = 0$ into the equation gives $0 = \Delta H - T\Delta S$, which rearranges to $\Delta H = T\Delta S$.
Final Answer: (c)