In this question, we have what is called Oxidizing agent, and this kind of agent is very common in reduction-oxidation reactions, where we have a shift of charges and electrons, some elements will gain electrons and become reduced and some elements will lose electrons and become oxidized, and we also have the agents, the oxidizing agent and reducing agent, the oxidizing agent is the one that causes another element to oxidize, therefore the oxidizing agent is the one who gained electrons and was reduced.