Which of the following compounds exhibits acidic nature?

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

Borax is actually made of two tetrahedra and two triangular units joined together and should be written as: $Na_2[B_4O_5(OH)_4] \cdot 8H_2O$. Consider the following statements about borax:
$a.$ Each boron atom has four $B-O$ bonds
$b.$ Each boron atom has three $B-O$ bonds
$c.$ Two boron atoms have four $B-O$ bonds while other two have three $B-O$ bonds
$d.$ Each boron atom has one $-OH$ group
Select correct statement$(s)$:

The reaction of $H_{3}N_{3}B_{3}Cl_{3}$ $(A)$ with $LiBH_{4}$ in tetrahydrofuran gives inorganic benzene $(B)$. Further,the reaction of $(A)$ with $(C)$ leads to $H_{3}N_{3}B_{3}(Me)_{3}$. Then,compounds $(B)$ and $(C)$ respectively,are

Identify the compounds $A$,$X$ and $Z$ in the following reactions :
$A + 2HCl + 5H_2O \to 2NaCl + X$
$X$ $\xrightarrow[370 \ K]{\Delta} HBO_2$ $\xrightarrow[> 370 \ K]{\Delta} Z$

Consider the compounds,$BCl_3$ and $CCl_4$. How will they behave with water? Justify.