Which diagram illustrates that matter is always conserved in a chemical reaction?

Only the third option has the same number of each size and color circles on either side of the arrow.

Explanation:

I believe it represents the dissolution of two molecules of hydrogen peroxide into two water molecules and an oxygen gas molecule.

The first one balances oxygen but changes sulfur with either carbon or nitrogen

The second one needs 5 oxygen gas pairs instead of a single one on the left to balance the right side. Represents oxidation of a propane molecule if given enough oxygen.

The fourth one has an extra nitrogen on the left side. The hydrogen is balanced. Making amonia.

conservation of mass

mass before something happens = mass afterwards