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

Which one of the following statements is not true regarding diborane $(B_2H_6)$?

In the following reaction,assign the state for the underlined atom for the product of complete hydrolysis at $R.T.$:
$H_4\underline{B}_2O_6 + H_2O \longrightarrow H_3BO_3 + H_2O_2$

Select the correct statements for diborane $(B_2H_6)$:
$1.$ Boron is approximately $sp^3$ hybridized.
$2.$ The $H-B-H$ bridge angle is $180^o$.
$3.$ There are two terminal $B-H$ bonds for each boron atom.
$4.$ There are only $12$ bonding electrons available.

$A$ certain salt $X$ gives the following results:
$(i)$ Its aqueous solution is alkaline to litmus.
$(ii)$ It swells up to a glassy material $Y$ on strong heating.
$(iii)$ When conc. $H_2SO_4$ is added to a hot solution of $X$,white crystals of an acid $Z$ separate out.
Identify $X, Y$ and $Z$ and write the equations for the reactions.

When orthoboric acid $(H_3BO_3)$ is heated,the residue left is: