Hot and concentrated solution of borax is treated with hydrochloric acid to give A and B. “B” on strong heating undergoes dehydration to give C. then “C” is

The hot and concentrated solution of borax on reacting with hydrochloric acid gives boric acid.
The reaction between borax and hydrochloric acid is shown below.
${\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7.10{\mathrm{H}}_2\mathrm{O}+2\mathrm{HCl}\to 4\mathrm{B}{\left(\mathrm{OH}\right)}_3+2\mathrm{NaCl}+5{\mathrm{H}}_2\mathrm{O}$

In this reaction, one mole of borax reacts with two mole of hydrochloric acid to form four mole of boric acid, two mole of sodium chloride and five mole of water.
The compound A is sodium chloride and the compound B is boric acid.
The compound B boric acid on strong heating undergoes a dehydration reaction.
The dehydration reaction is defined as the reaction where removal of water takes place on heating the reactant solution strongly.
The dehydration reaction is shown below.

$\mathrm{B}{\left(\mathrm{OH}\right)}_3\stackrel{∆}{\to }{\mathrm{B}}_2{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}$

The compound boric acid on heating forms boron oxide and water as a product.
The compound C is boron oxide.
Boron oxide is an acidic oxide as on dissolving in water it forms an acid known as boric acid.