In a reaction, 2H_2S + SO_2 arrow 2H_2O + 3S Identify the substance...

The given reaction is $2\text{H}_2\text{S} + \text{SO}_2 \rightarrow 2\text{H}_2\text{O} + 3\text{S}$.

The oxidation state of sulphur in $\text{H}_2\text{S}$ is $-2$.

The oxidation state of sulphur in $\text{SO}_2$ is $+4$.

The oxidation state of elemental sulphur ($\text{S}$) is $0$.

The oxidation state of sulphur increases from $-2$ in $\text{H}_2\text{S}$ to $0$ in $\text{S}$. An increase in oxidation state corresponds to oxidation.

The oxidation state of sulphur decreases from $+4$ in $\text{SO}_2$ to $0$ in $\text{S}$. A decrease in oxidation state corresponds to reduction.

Thus, $\text{H}_2\text{S}$ is oxidized and $\text{SO}_2$ is reduced.

Answer: $\text{H}_2\text{S}$