Which of the following does not exist in free form?

Which of the following is not associated with water molecules?

$BCl_3$ exists as a monomer,whereas $AlCl_3$ is dimerized through halogen bridging. Give reason. Explain the structure of the dimer of $AlCl_3$ also.

When borax is heated with $CoO$ on a platinum loop,the blue coloured bead formed is largely due to

The correct statements from the following are $:$
$(A)$ $Tl^{3+}$ is a powerful oxidising agent
$(B)$ $Al^{3+}$ does not get reduced easily
$(C)$ Both $Al^{3+}$ and $Tl^{3+}$ are very stable in solution
$(D)$ $Tl^{+}$ is more stable than $Tl^{3+}$
$(E)$ $Al^{3+}$ and $Tl^{+}$ are highly stable
Choose the correct answer from the options given below $:$

Borazine,also known as inorganic benzene,can be prepared by the reaction of $3$ equivalents of "$X$" with $6$ equivalents of "$Y$". "$X$" and "$Y$",respectively,are: