Step 1: Read the statement.
The given line is, mass can neither be created nor destroyed. We must name the law it describes.
Step 2: Recall each law briefly.
Gay Lussac's law is about volumes of reacting gases. The law of definite proportion is about fixed mass ratios in a compound. The law of multiple proportions compares masses when two elements form more than one compound.
Step 3: Match the wording.
The idea that total mass stays the same and is never made or lost is exactly the law of conservation of mass.
Step 4: Know who gave it.
This law was stated by Antoine Lavoisier. It says the total mass of a closed system stays constant.
Step 5: Apply the meaning.
In any chemical change the total mass of reactants equals the total mass of products. Atoms only rearrange, none vanish or appear.
Step 6: Pick the answer.
The law is the law of conservation of mass, which is option 3.
\[ \boxed{\text{Law of conservation of mass}} \]