Borax is a very important compound of Boron. It finds a lot of applications. Its formula is

• A

  ${\mathrm{Na}}_2\left[{\mathrm{B}}_4{\mathrm{O}}_5{\left(\mathrm{OH}\right)}_4\right].4{\mathrm{H}}_2\mathrm{O}$

• B

  ${\mathrm{Na}}_2\left[{\mathrm{B}}_4{\mathrm{O}}_5{\left(\mathrm{OH}\right)}_4\right].8{\mathrm{H}}_2\mathrm{O}$

• C

  ${\mathrm{Na}}_2\left[{\mathrm{B}}_4{\mathrm{O}}_5{\left(\mathrm{OH}\right)}_4\right].3{\mathrm{H}}_2\mathrm{O}$

• D

  ${\mathrm{Na}}_2\left[{\mathrm{B}}_4{\mathrm{O}}_5{\left(\mathrm{OH}\right)}_4\right].5{\mathrm{H}}_2\mathrm{O}$