Which of the following boron compounds is the hardest?

$H_3BO_3$ $\xrightarrow{T_1} X$ $\xrightarrow{T_2} Y$ $\xrightarrow{\text{red hot}} B_2O_3$
If $T_1 < T_2$,then $X$ and $Y$ respectively are:

Inorganic benzene is

Group-$13$ elements react with $O_2$ to form oxides of the type $M_2O_3$ ($M =$ element). Which among the following is the most basic oxide?

Which of the following compounds exhibits acidic nature?

Boron halides are Lewis acids because.....