Magnesium is burnt in the air to form A and B . When B is hydrolyzed, C and D are formed. D is the reactant…

In air, oxygen and nitrogen gases are present with which magnesium reacts and the reaction takes place as below: 

$$\begin{gathered} \mathrm{Mg}\left(\mathrm{s}\right) + \mathrm{Air} \to \mathrm{MgO} + {\mathrm{Mg}}_3{\mathrm{N}}_2 \\ \mathrm{A} \mathrm{B} \end{gathered}$$

B reacts with water to form magnesium hydroxide and ammonia.

${\mathrm{Mg}}_3{\mathrm{N}}_2 + {\mathrm{H}}_2\mathrm{O} \to \mathrm{Mg}{\left(\mathrm{OH}\right)}_2 + {\mathrm{NH}}_3$

${\mathrm{NH}}_3$ acts as a reactant in the manufacture of nitric acid by Ostwald's process.