When the gases sulphur dioxide and hydrogen sulphide mix in the presence of water , the reaction $SO_2+2H_2S \to 2H_2O +3S$ Occurs. Here Hydrogen sulphide is acting as

Question

$\begin{array}{1 1} \text{a. An oxidising agent} \\ \text{ b. A reducing agent} \\ \text{ c. A dehydrating agent} \\ \text{ d. A catalyst} \end{array}$