According to balanced chemical equation 212 g (2 moles) of NaClO3 on complete consumption produces 96 g (3 moles) of O2. So, using this as a reference point the amount of O2 produced by consuming 12.5 g of NaClO3 will be calculated as,
212 g of NaClO3 produced / 12.5 g NaClO3 will produce = 96 g of O2 / x
Solving for x,
x = 12.5 g /212 g x 96 g
x = 5. 66 g
So, the correct answer is A.