To demonstrate that matter is conserved during a chemical change or reaction, the best procedure from your options would be:
Find the mass of an acid and a base. Combine them to produce water and a salt. Find the mass of the water and salt.
This procedure involves a chemical reaction where the reactants (acid and base) produce products (water and salt). By measuring the mass of both the reactants before the reaction and the products after the reaction, you can show that the total mass remains the same, thus demonstrating the law of conservation of mass.