Aqueous solution containing 1 mol of borax reacts with 2 mol of acids. This is because of

formation of 2 mol of $\mathrm{B}{\left(\mathrm{OH}\right)}_3$ only

formation of 1 mol each of $\mathrm{B}{\left(\mathrm{OH}\right)}_3$ and ${\left[\mathrm{B}{\left(\mathrm{OH}\right)}_4\right]}^{-}$

formation of 2 mol of ${\left[\mathrm{B}{\left(\mathrm{OH}\right)}_4\right]}^{-}$ only

formation of 2 mol each of ${\left[\mathrm{B}{\left(\mathrm{OH}\right)}_4\right]}^{-}$ and $\mathrm{B}{\left(\mathrm{OH}\right)}_3$, of which only ${\left[\mathrm{B}{\left(\mathrm{OH}\right)}_4\right]}^{-}$ reacts with acid