Addition of mineral acid to an aqueous solution of Borax, orthoboric acid is formed.

$\mathrm{Na}_{2} \mathrm{B}_{4} \mathrm{O}_{7}+2 \mathrm{HCl}+5 \mathrm{H}_{2} \mathrm{O} \rightarrow 4 \mathrm{H}_{3} \mathrm{BO}_{3}+2 \mathrm{NaCl}$

Hence, the correct answer is option B.