Question

What feature of a chemical equation represents the law of conservation of matter?

Answer 1

Answer is: balanced equation.
Law of conservation of matter claims that in any system that is closed to the transfer of matter, the amount of matter in the system stays constant. 
Balanced equation states that same number and type of atoms after the chemical reaction must present as before the chemical change.

Answer 2

The idea that, on each side of the equation (reactants and products), there are the same numbers of atoms of each constituent element shows that matter is conserved. There is the same mass of reactants at the beginning of an equation as there are products when the reaction completes.