Explanation

(a)
$B(OH)_{3} + NaOH \rightleftharpoons NaBO_{2} + {Na}^{+}\left\lbrack B(OH)_{4} \right\rbrack^{-} + H_{2}O$
This reaction is reversible reaction because sodium metaborate, $Na^{+}\left\lbrack B(OH)_{4} \right\rbrack^{-}$ formed by the reaction between $B(OH)_{3}$ and $NaOH$ gets hydrolysed to regenerate $B(OH)_{3}$ and $NaOH$.
$Na^{+}\left\lbrack B(OH)_{4} \right\rbrack^{-}\overset{\ \ Hydrolysis\ \ }{\rightarrow}NaOH + B(OH)_{3}$
If some quantity of polyhydroxy compounds like $cis$-1, 2-diol, catechol, glycerol etc is added to the reaction mixture then the $B(OH)_{3}$combines with such polyhydroxy compounds to give chelated complex compound. Due to complex compound formation, stability increases and due to higher stability of complex, reaction moves in forward direction.