Although mass is a property of matter we can conveniently measure in the laboratory, the coefficients of a balanced chemical equation are not directly interpreted on the basis of mass. explain why.

The coefficients of a balanced chemical equation means number of particles (molecules or unit formulas) not mass.

The law of definite proportion states that a compound is constituted by the same elements in the same proportion regardless the total amount.

A chemical formula states the ratio of the atoms in a compound as number of atoms, not as mass or weight.

For example, the formula Na₂O means that the compound is formed in a ratio of 2 atoms of Na to 1 atom of O. (Not two grams of Na to one gram of O).

Then, the chemical equation Na + O₂ → Na₂O when balanced results in

4Na + O₂ → 2Na₂O.

The coefficients are used to balance the number of atoms in the compounds of each side: 4 atoms of Na in the reactant side and 4 atoms of Na in the product side, 2 atoms of O in the reactant side and 2 atoms of O in the reactant side.

So, the coefficients mean number of unit formulas, not mass.

what are the choices?

it is the most commonly accepted one