Identify the processes in which the entropy of the system...

A. Freezing of water to ice at $273 \text{ K}$: The process involves the conversion of a liquid to a solid. The molecules become more ordered, so the entropy of the system decreases ($\Delta S < 0$).

B. $2\mathrm{SO}_{2(\mathrm{g})} + \mathrm{O}_{2(\mathrm{g})} \rightarrow 2\mathrm{SO}_{3(\mathrm{g})}$: The number of moles of gaseous species decreases from $3$ to $2$. This leads to a decrease in randomness, so the entropy decreases ($\Delta S < 0$).

C. Mixing of two non-reacting ideal gases at constant temperature: The volume available for each gas increases, leading to an increase in randomness. Thus, the entropy of the system increases ($\Delta S > 0$).

D. Dissolution of oxygen gas in water: The gaseous oxygen molecules lose their freedom of movement when they dissolve in a liquid solvent. This results in a decrease in randomness, so the entropy decreases ($\Delta S < 0$).

Therefore, the entropy of the system decreases in processes A, B, and D.

Answer: A, B and D