Why is measuring less mass after gas escapes from an open container not against conservation of mass?

Correct answer: Because mass of escaped gas was not measured

Conservation of mass applies to a complete, closed system in which every reactant and product is included. In an open container, the gas formed during the reaction escapes into the surroundings, so the balance or measurement records only the material left in the container. The measured mass decreases, but the escaped gas still has mass and has not been destroyed. Therefore option A correctly explains the apparent loss.