Magnesium ribbon is rubbed before burning because it has a coating of
(a) basic magnesium carbonate
(b) basic magnesium oxide
(c) basic magnesium sulphide
(d) basic magnesium chloride

Dear Student,
Magnesium ribbon is a very reactive metal. When stored it reacts with oxygen to form a layer of magnesium oxide on its surface. This layer of magnesium oxide being a stable compound prevents further reaction of magnesium with oxygen. The magnesium is cleaned before burning in air to remove this layer so that the metal can be exposed to air properly.

Hence, option b basic magnesium oxide is correct.
Regards