Final answer:
Mn in MnO₂ is reduced because its oxidation state decreases from +4 in MnO₂ to +2 in MnCl₂ in the reaction 4HCl + MnO₂ → Cl₂ + 2H₂O + MnCl₂.
Step-by-step explanation:
Identifying the Reduced Substance
In the reaction 4HCl + MnO₂ → Cl₂ + 2H₂O + MnCl₂, the substance that is reduced is Mn in MnO₂ (option b). This can be determined by analyzing the changes in oxidation states of the elements involved in the reaction. Chlorine (Cl) goes from an oxidation state of -1 in HCl to 0 in Cl₂, which means Cl is oxidized. Manganese (Mn) goes from +4 in MnO₂ to +2 in MnCl₂, indicating a reduction in its oxidation state, and hence Mn is reduced. Hydrogen (H) and Oxygen (O) do not undergo any change in their oxidation states relevant to the question of which substance is reduced.