Consider the following equilibrium for the synthesis of sulfur...

The given reaction is $2\mathrm{SO}_2(\mathrm{g}) + \mathrm{O}_2(\mathrm{g}) \rightleftharpoons 2\mathrm{SO}_3(\mathrm{g})$ with $\Delta H = -198 \text{ kJ mol}^{-1}$.

According to Le Chatelier's principle:

The reaction is exothermic ($\Delta H < 0$). Lowering the temperature favors the forward reaction, increasing the yield of $\mathrm{SO}_3$. Thus, condition A is correct.

The number of moles of gaseous reactants is $3$ and gaseous products is $2$. Since the forward reaction proceeds with a decrease in the number of moles of gas, increasing the pressure shifts the equilibrium in the forward direction. Thus, condition C is correct.

Adding a reactant ($\mathrm{O}_2$) increases its concentration, which shifts the equilibrium in the forward direction to consume the added reactant. Thus, condition D is correct.

Therefore, conditions A, C, and D favor the maximum yield of $\mathrm{SO}_3$.

Answer: A, C and D only