If gas escapes from an open bottle during reaction what is the correct comment on conservation of mass?

Correct answer: Measurement is incomplete because gas was not counted

The governing concept is conservation of mass in a closed system. The law states that the total mass before a reaction equals the total mass after it when every reactant and product is included. In an open bottle, some gaseous product escapes, so the measured mass after reaction appears smaller because that gas is no longer on the balance. The law has not failed; the observation is incomplete. Thus option B is correct. Option A wrongly rejects the law, option C falsely claims that gas has no mass, and option D confuses loss from measurement with failure to form products.