The oxidation of ${SO}_{2}$ to $H_{2} {SO}_{4}$ in acid rain is thought to occur by the following mechanism
$\begin{array}{l} {SO}_{2} (a q) + 2 H_{2} O (l) ⟶ {HSO}_{3}^{-} (a q) + H_{3} O^{+} (a q) \\ 2 {HSO}_{3}^{-} (a q) + O_{2} (a q) ⟶ S_{2} {O_{7}}^{2 -} (a q) + H_{2} O (l) \\ S_{2} {O_{7}}^{2} - (a q) + 3 H_{2} O (l) ⟶ 2 {SO}_{4}^{2 -} (a q) + 2 H_{3} O^{+} (a q) \end{array}$
Which species in this mechanism can be given the following designations?

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Reactant	Catalyst	Intermediate
${SO}_{2} (a q)$	$H_{2} O (l)$	${HSO}_{3}^{-} (a q), H_{3} O^{+} (a q)$

Reactant	Catalyst	Intermediate
${SO}_{2} (a q)$	${HSO}_{3}^{-} (a q)$	$S_{2} O_{7}^{2 -} (a q)$

Reactant	Catalyst	Intermediate
${SO}_{2} (a q), H_{2} O (l)$	$S_{2} O_{7}^{2 -} (a q)$	${HSO}_{3}^{-} (a q)$

Reactant	Catalyst	Intermediate
${SO}_{2} (a q), H_{2} O (l)$	none	${HSO}_{3}^{-} (a q), S_{2} O_{7}^{2 -} (a q)$

answer is D.

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Detailed Solution

The mechanism of the reaction is;

$\begin{array}{l} {SO}_{2} (a q) + 2 H_{2} O (l) ⟶ {HSO}_{3}^{-} (a q) + H_{3} O^{+} (a q) \\ 2 {HSO}_{3}^{-} (a q) + O_{2} (a q) ⟶ S_{2} {O_{7}}^{2 -} (a q) + H_{2} O (l) \\ S_{2} {O_{7}}^{2} - (a q) + 3 H_{2} O (l) ⟶ 2 {SO}_{4}^{2 -} (a q) + 2 H_{3} O^{+} (a q) \end{array}$

As, it can be seen here the reactant is ${SO}_{2} (a q)$ and $H_{2} O (l) c$ . There is no catalyst used in this process. ${HSO}_{3}^{-} (a q), S_{2} O_{7}^{2 -} (a q)$ are the intermediate of this process.