An aluminum patio chair reacts with the oxygen in the air. a layer of aluminum oxide forms on the chair. the formation of aluminum oxide is both an oxidation-reduction reaction and a reaction. a. single-replacement b. combustion c. decomposition d. synthesis

D synthesis reaction

Single replacement will be like
Cl2 + 2NaBr —> 2NaCl + Br2
This is because only one element is moved from the reactants to the products (Cl)

Combustion will be like
CH4 + 2O2 —> 2H2O + CO2
Which involves oxygen gas

Decomposition is
NaCl —> Na + Cl
Where the compound breaks apart

And synthesis is
Na + Cl—> NaCl
Where a compound is made
Much like your problem
Al + O2 —> Al2O3 (aluminum oxide)

The mass has escaped into a gas which you can not see so the law stays true.