To demonstrate that matter is conserved during a chemical change or reaction, the best procedure would be:
Find the mass of an acid and a base, combine them to produce water and a salt, and then find the mass of the water and salt.
This experiment can effectively demonstrate the law of conservation of mass, which states that mass is neither created nor destroyed in a chemical reaction. By measuring the mass of the reactants (the acid and the base) before the reaction and then measuring the mass of the products (water and salt) after the reaction, you should find that the total mass remains the same before and after the reaction.
The other examples, such as burning a log or melting butter, involve procedures that can demonstrate changes in state or form, but they may not effectively illustrate the conservation of mass as clearly as a chemical reaction would. Burning a log results in gases (smoke) that escape, making it harder to measure mass accurately in the same closed system. Melting butter is a physical change and does not involve a chemical reaction. Tearing paper involves a physical change as well. Thus, the reaction between the acid and base is the most relevant to your goal of showing that matter is conserved during a chemical change.