Which of the following compounds of boron does not exist in the free form?

Given below are two statements:
Statement $I$: Group $13$ trivalent halides get easily hydrolyzed by water due to their covalent nature.
Statement $II$: $AlCl_3$ upon hydrolysis in acidified aqueous solution forms octahedral $[Al(H_2O)_6]^{3+}$ ion.
In the light of the above statements,choose the correct answer from the options given below:

In the structure of $B_2H_6$,the number of $BH_2$ groups present in one plane,and the number of $B-H$ terminal bonds,$B-B$ bonds,and $B-H-B$ bridge bonds are respectively:

The stability of $+1$ oxidation state among $Al$,$Ga$,$In$ and $Tl$ increases in the sequence:

Is boric acid a protic acid? Explain.

Aluminium reacts with caustic soda to form