The correct option is A 6OH−+4S→2S2−+S2O2−3+3H2O
The unbalanced redox equation is
S+OH−→S2−+S2O2−3
Balance all atoms other than H and O.
3S+OH−→S2−+S2O2−3
The oxidation number of S changes from 0 to -2. The change in the oxidation number of S is 2.
The oxidation number of S changes from 0 to 4. The change in the oxidation number of S is 4.
The increase in the oxidation number is balanced with decrease in the oxidation number by multiplying
S and S2− with 2.
4S+OH−→2S2−+S2O2−3
O atoms are balanced by adding 2 water molecules on LHS.
4S+OH−+2H2O→2S2−+S2O2−3
Hydrogen atoms are balanced by adding 5 H+ atoms on RHS
4S+OH−+2H2O→2S2−+S2O2−3+5H+.
Since the reaction occurs in basic medium, 5 hydroxide ions are added on the either side
4S+OH−+2H2O+5OH−→2S2−+S2O2−3+5H++5OH−.
On RHS five protons and five hydroxide ions forms five water molecules out of which two cancels out with two water molecule on RHS.
6OH−+4S→2S2−+S2O2−3+3H2O
This is the balanced chemical equation.