The correct option is A MgCO3⟶MgO+CO2
The reactions are as follows:
MgCO3⟶MgO+CO2
+2 +2
O2+2H2⟶2H2O
0 0 +1 -2
K+H2O⟶KOH+(1/2)H2
0 +1 0
MnBr3⟶MnBr2+(1/2)Br2
+3 +2 0
As intermolecular redox reaction and redox reactions are same, only A is not a redox reaction.