Question

Find the number of proton donor acid(s) from the following:
$B(OH{)}_3,Mg(OH{)}_2,Si(OH{)}_4,S{O}_2(OH{)}_2,Ba(OH{)}_2$ ___

Answer 1

First, let us get the tricky ones out of our way.
$B(OH{)}_3$ is boric acid. It is not a proton donor but rather a hydroxide acceptor.
It is a weak electrolyte and in water, gets the latter to release protons via
accepting $O{H}^{-}$.

Magnesium hydroxide and barium hydroxide are basic. They are hydroxide donors.
What is left is silicic acid and sulphuric acid. The former is only of academic interest.
But it is a very weak acid with a $p{K}_{a1}=9.84$.