Why does magnesium gain weight on burning?
(A) It combines with nitrogen
(B) It combines with oxygen
(C) Absorbs massive amount of heat
(D) Absorbs massive amount of air while burning

Hint: Magnesium is a chemical element with atomic number $12$. It is a shiny gray solid which bears a close physical resemblance to the other five elements in the second column of the periodic table. It is one of the seven essential macro minerals. It reacts with oxygen and gives a powdery product.

Complete step by step answer:
Magnesium is the most chemically active element. It is labelled as the eighth most abundant element and about $2\% $ of the Earth’s crust is constituted by Magnesium.
Moreover, the place of hydrogen is taken by Magnesium and a number of metals can be produced using thermal reduction of its salt. Since it was found in the bonded form in nature, Magnesium took so long to isolate.
Now, it gains weight on burning. The increase in mass is due to the fact that oxygen (from the air) combines with magnesium to give magnesium oxide. So, the balanced chemical equation is:
$2Mg + {O_2} \to 2MgO$
Oxygen and magnesium come together in a chemical reaction to form this compound. After burning, it forms a white powder of magnesium oxide and magnesium gives up two electrons to oxygen atom to form this powdery product. Overall, this is an exothermic reaction.

Hence, option B and D are correct.

Note: Magnesium is used to treat skin related problems, anxiety, deficit hyperactivity disorder etc. The green leafy vegetables, whole grains, seeds and nuts are a great source of magnesium. Moreover, the flashbulbs also contain magnesium. When Mg burns, it produces a bright light that helps the photographer to take photos in the dark.