Explanation

(a)
Central boron atom in $H_{3}BO_{3}$ is electron deficient, therefore it accepts a pair of electron, hence it is weak Lewis acid. There is no $d$-orbital of suitable energy in boron atom. So, it can accommodate only one additional electron pair in its outermost shell. Thus, $H_{3}BO_{3}$ is a monobasic weak Lewis acid.
$H_{2}O + B(OH)_{3} \longrightarrow \left\lbrack B(OH)_{4} \right\rbrack^{-} + H^{+}$
base acid