1) Octane burns in air to form carbon dioxide and H2O.
2C8H18 (l) + 25O2 (g) ----> 16CO2 (g) + 18H2O (l)
2) Zinc oxide reacts with excess carbon.
2ZnO (s) +C (s) ----> 2Zn (s) + CO2 (g)
3) Sodium hydroxide and magnesium. [ This reaction doesn't occur since sodium is more reactive than magnesium and hence it a displacement reaction won't occur but I'll just provide you with what would happen if it was possible ]
2NaOH (aq) + Mg (s) ----> Mg(OH)2 (s) + 2Na (s)