Identify from following physical transformations that exhibits decrease in entropy.

Entropy change depends on the transition between physical states. Gases have the highest entropy, followed by liquids, then solids with the lowest entropy due to increasing disorder.

Option A shows melting: $\mathrm{H}_2\mathrm{O}_{(\mathrm{s})} \longrightarrow \mathrm{H}_2\mathrm{O}_{(\ell)}$, where entropy increases ($\Delta S > 0$).

Option B shows vaporization: $\mathrm{H}_2\mathrm{O}_{(\ell)} \longrightarrow \mathrm{H}_2\mathrm{O}_{(\mathrm{g})}$, where entropy increases significantly ($\Delta S > 0$).

Option C shows sublimation: $\mathrm{H}_2\mathrm{O}_{(\mathrm{s})} \longrightarrow \mathrm{H}_2\mathrm{O}_{(\mathrm{g})}$, where entropy increases ($\Delta S > 0$).

Option D shows condensation: $\mathrm{H}_2\mathrm{O}_{(\mathrm{g})} \longrightarrow \mathrm{H}_2\mathrm{O}_{(\ell)}$, where disorder decreases, so $\Delta S < 0$.

The transformation exhibiting decreased entropy is condensation.

Final answer: $\boxed{D}$