Answer:
The correct answer is:
K2Cr2O7, because the oxidation number of Cr changes from +6 to +3.
Step-by-step explanation:
In this reaction, K2Cr2O7 is the reactant that acts as an oxidizing agent. An oxidizing agent is a substance that causes another substance to be oxidized (to lose electrons). In this case, the oxidation number of Cr in K2Cr2O7 changes from +6 to +3 as it is oxidized by HCl. This indicates that K2Cr2O7 is acting as the oxidizing agent in the reaction.
HCl is not acting as an oxidizing agent in this reaction. The oxidation number of H in HCl does not change, and the oxidation number of Cl changes from -1 to 0, which indicates that it is being reduced (gaining electrons).
It's important to note that the concept of oxidation numbers is used to keep track of the electrons that are transferred during a chemical reaction. The oxidation number of an element in a compound is a measure of the degree to which it has been oxidized or reduced. In general, an element that has a higher oxidation number before a reaction will have a lower oxidation number after the reaction, indicating that it has been reduced, while an element with a lower oxidation number before the reaction will have a higher oxidation number after the reaction, indicating that it has been oxidized.