The correct option is
C Cr2O2−7+3C2H4O+8H+→3C2H4O2+2Cr3++4H2OThe unbalanced redox equation is as follows:
Cr2O2−7+C2H4O+H+→C2H4O2+Cr3+
Balance all atoms other than H and O.
Cr2O2−7+C2H4O+H+→C2H4O2+2Cr3+
The oxidation number of Cr changes from 6 to 3. So the change in oxidation number of one Cr atom is 3.
For 2 Cr atoms, the oxidation number changes by 6.
The oxidation number of C changes from -1 to 0. So the change in oxidation number of 1 C atom is 1.
For 2 C atoms, the oxidation number changes by 2.
The increase in the oxidation number is balanced with decrease in the oxidation number by multiplying
C2H4O and C2H4O2 with 3.
Cr2O2−7+3C2H4O+H+→3C2H4O2+2Cr3+
O atoms are balanced by adding 4 water molecules on RHS.
Cr2O2−7+3C2H4O+H+→3C2H4O2+2Cr3++4H2O
Hydrogen atoms are balanced by adding 7 H+ atoms on RHS.
Cr2O2−7+3C2H4O+8H+→3C2H4O2+2Cr3++4H2O
This is the balanced chemical equation.