The correct option is C Remove magnesium oxide formed over the surface of magnesium
When exposed to air many metals will slowly react with oxygen in the atmosphere to form metal oxides and magnesium is one of those metals. You are probably familiar with rust which is simply the oxide that forms when iron is exposed to oxygen in the air. With magnesium, the oxide layer forms a white coating on the outside of the metal that will slow down or interfere with the burning process.The ribbons should be rubbed with a fine sandpaper, to remove the layer. Burning magnesium results in magnesium oxide and also some magnesium nitride (reaction with nitrogen in the air). Burning speeds up the conversion of magnesium to magnesium oxide in air.