crutudg
H in HCl is +1
Cl in HCl is -1
Mn in MnO2 is +4
O in MnO2 is -2
Cl in Cl2 is 0
H in H2O is +1
O in H2O is -2
Mn in MnCl2 is +2
Cl in MnCl2 is -1
so..
SOME of the Cl went from -1 to 0 and was "oxidized".. increased in charge
SOME of the Cl stayed the same
All of the Mn went from +4 to +2 and was "reduced"
Reduction means gaining electrons and therefore becoming more negative, so it must be the H (the Mg is oxidized).