Among the given compounds $Mg ( OH )_{2}$ is weakly basic, $Al ( OH )_{3}$ and $Be ( OH )_{2}$ are amphoteric. Only $B ( OH )_{3}$ is acid. It is a weak monobasic acid - Boric acid $\left( H _{3} BO _{3}\right)$. Also it is Lewis acid and not a proton donar (Bronsted acid). $B ( OH )_{3}$ accept lone pair of electrons from water molecule .
In this species $B ^{3+}$ due to its very small size polarize $O$-atom upto greater extent and attracts its electron cloud towards itself.
Hence, $O$ atom in turn pull the bonding electrons of $O-H$ bond thereby making this bond weak. Thus this species has tendency to loose proton.
$B ( OH )_{3}+ H _{2} O \longrightarrow \left[ B ( OH )_{4}\right]^{-}+ H ^{+}$
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