When the gases sulphur dioxide and hydrogen sulphide mix in the presence of water, the reaction #"SO"_2 + 2"H"_2"S" -> 2"H"_2"O" + 3"S"# occurs. Here hydrogen sulphide is acting as?
#a)# an oxidizing agent
#b)# a reducing agent
#c)# a dehydrating agent
#d)# a catalyst
Hydrogen sulphide is a reducing agent.
A good starting point for any reaction is to look at oxidation numbers. If they change, a redox reaction has occurred.
Here are some thoughts.
A dehydrating agent will extract water from another reactant. I would say that you need a hydroxyl (OH) group for this to occur. A dehydrating agent could be an acid and protonate the OH group, which can then leave as water. A catalyst is not used up in the reaction, so is hydrogen sulphide used up?
Hydrogen is usually in a +1 oxidation state and oxygen a -2 state.