The variable that is not conserved during the combustion reaction of propane as shown by the equation would be none of the above.
Recall that the law of conservation says that atom/matter/mass is conserved in the course of reactions, although forms might change.
The number of moles of each atom is conserved because both the reactant and product sides have equal numbers of each atom that make up the compounds.
Unless the reaction is not carried out in an isolated system, the mass should be conserved. In other words, the total mass of the reactants must be equal to the total mass of the products. If the system is not isolated, materials may be exchanged with the surroundings and this might make the mass not to be conserved.
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