Answer:- (A) Metaborate

On heating, borax loses water of crystallisation and swells up to form fluffy mass. On further heating, it melts to give a clear liquid which solidifies to a transparent glossy bead consisting of sodium metaborate and boric unhydride.

$\mathrm{Na}_{2} \mathrm{B}_{4} \mathrm{O}_{7} \cdot 10 \mathrm{H}_{2} \mathrm{O} \stackrel{\Delta}{\longrightarrow} \mathrm{Na}_{2} \mathrm{B}_{4} \mathrm{O}_{7} \stackrel{\Delta}{\longrightarrow}{\underset { Sodium Metaborate }{2 \mathrm{NaBO}_{2}}}+\mathrm{B}_{2} \mathrm{O}_{3}$

Thus, Metaborate is formed.