Which of the following halides of group $13$ elements is the most acidic?

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

The correct order of atomic radii of group $13$ elements is

Substance $X$ reacts with an aqueous solution of $NaOH$ to form $Y$ and releases $H_2$ gas. When an 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 $1200^{\circ}C$ produces $Al_2O_3$. Identify $X, Y,$ and $Z$.

Diborane reacts with ammonia under different conditions to give a variety of products. Which one among the following is not formed in these reactions?

How would you explain the lower atomic radius of $Ga$ as compared to $Al$?