${\mathrm{Ca}}_2{\mathrm{B}}_6{\mathrm{O}}_{11}+{\mathrm{Na}}_2{\mathrm{CO}}_3\stackrel{\mathrm{\Delta }}{⟶}\left[\mathrm{X}\right]+{\mathrm{CaCO}}_3+{\mathrm{NaBO}}_2$

(unbalanced equation) The incorrect choice for [X] is:

The structure of anion of crystalline [X] has one boron atom ${\mathrm{sp}}^3$ hybridized and the other three boron atoms ${\mathrm{sp}}^2$ hybridized.

X on heating gives a compound that in reaction with hydrogen peroxide in an alkaline medium yields a compound used as a brightener in soaps.

Its aqueous solution is basic.

$\left[\mathrm{X}\right]+\text{ conc. }{\mathrm{H}}_2{\mathrm{SO}}_4⟶{\mathrm{H}}_3{\mathrm{BO}}_3+{\mathrm{Na}}_2{\mathrm{SO}}_4$.