(d) MnO₂ is reduced and HCl is oxidised.
In this reaction, Mn in MnO₂ goes from +4 to +2 (gains electrons → reduction), while Cl in HCl goes from −1 to 0 in Cl₂ (loses electrons → oxidation).
In a redox reaction, both oxidation and reduction must occur simultaneously. Check oxidation states: Mn drops from +4 to +2 (reduced), and Cl rises from −1 to 0 (oxidised). Options (a), (b), and (c) each describe only one half — only (d) correctly identifies both changes, making it the complete and correct answer.