Which of the following boron compounds is the hardest?

Describe the general trends in the following properties of the elements in Groups $13$ and $14$.
$(A)$ Atomic size
$(B)$ Ionization enthalpy
$(C)$ Metallic character
$(D)$ Oxidation states
$(E)$ Nature of halides

An aqueous solution of borax is

On adding ammonium hydroxide solution to $Al_2(SO_4)_3$ (aq) :

Aluminium $(III)$ chloride forms a dimer because ........

Which one of the following is the correct statement?