Answer;
-H2O, water
Explanation;
-Hydrocarbons undergoes combustion to form water and carbon dioxide. Propane (C3H8) is a hydrocarbon that belongs to the homologous series alkanes. Thus, propane will undergo combustion to form 3 molecules of carbon dioxide and 4 molecules of water.
Thus the complete equation will be;
-C₃H₈ + 5O₂ = 3CO₂ + 4H₂O