Answer:
Cr(OH)4^-
Step-by-step explanation:
Reducing agents are elements or compounds which loses (or "donates") an electron to an electron recipient (oxidizing agent) in a redox chemical reaction. A reducing agent is thus oxidized when it loses electrons in the redox reaction.
A reducing agent is a substance that causes another substance to be reduced. Hence for us to identify an reducing agent, we simply look at the oxidation number of a specie before and after the redox reaction. If the oxidation number is greater in the product, then it lost electrons and the substance is the reducing agent.
If we look from left to right, the oxidation number of chromium increased from +3 to +6. Hence the specie Cr(OH)4^- is the reducing agent in the reaction.