Which of the following boron compounds is the hardest?

The correct formula of borax is

The incorrect stability order of $+3$ and $+1$ states of $13^{th}$ group elements (boron family) is

Identify the correct statements from the following:
$i$. $H_3BO_3$ is a monobasic acid.
$ii$. The correct formula of borax is $Na_2[B_4O_5(OH)_4] \cdot 8H_2O$.
$iii$. $NaBH_4$ is a reducing agent.

Which of the following statements is correct?

$X$ reacts with aqueous $NaOH$ to form $Y$ and releases $H_2$ gas. When the aqueous solution of $Y$ is heated at $323 \ K - 333 \ K$ and $CO_2$ gas is passed through it,it yields $Al_2O_3 \cdot xH_2O$ (often represented as $Al(OH)_3$) and $Z$. Heating $Z$ at $1200 \ ^\circ C$ gives $Al_2O_3$. Identify $X, Y,$ and $Z$.