chem MC

To clarify it should be written as
aCl2 + 6OH - → bCl - + ClO3 - + 3H2O
not aCl2 + 6OH - → bCl - + ClO 3- + 3H2O
(I know how uneasy to work with limited editing tools -_-!)

Then start to check:
Atomic balance (Cl in this case): as long as all choices follows b=2a-1; so it does not help you to distingish choices.

And then check:
Charge balance (or electron balance): 6 known -ve charge on the left, 1 known -ve charge on right; Cl2 number does not affect charge, the simplest way is to change number of Cl - ion balance the charge, the result is obviously b=5 here. Thus C is the answer.

I think I won't give you the answer, but I hope that you need to consider the charge balance of the equation because there are six negative in the left hand side, so you need to made up the number of total charge in the right hand side be negative six!! you see what I meant?? put the number into a & b one by one..... you will see which one you can balance the charge!!

Hope this can help you.