During which of the following is mass conserved?

Mass is conserved in a chemical reaction when the total mass of the reactants is equal to the total mass of the products. This principle is based on the law of conservation of mass, which states that matter cannot be created or destroyed in a closed system.

In all chemical reactions, the atoms present in the reactants are rearranged to form products, but no atoms are lost or gained in the process, thus ensuring that the mass before and after the reaction remains constant. This applies to both exothermic and endothermic reactions, as well as to physical changes, but the key aspect is that mass conservation holds true for all chemical transformations.

By contrast, combustion reactions, while they do conserve mass, represent a specific type of chemical reaction. The same applies to exothermic reactions, which release energy but still maintain the conservation of mass principle. Therefore, the comprehensive nature of chemical reactions makes this the correct choice, as it encapsulates the idea that mass conservation applies universally in this context.