Why can a lower measured mass after gas escapes from an open container give a wrong conclusion?

Correct answer: Because the gas mass is not included in the later measurement.

The conservation-of-mass law applies to a complete, closed system. In an open container, the gas produced may escape into the surroundings before the final weighing. The balance then measures only the remaining material, not the escaped gas, so the observed mass is lower even though total mass has been conserved. Therefore A is correct. Mass is not destroyed, the reaction need not stop, and a chemical reaction does not become physical merely because gas escapes.