Which of the following is the most basic oxide?

Which of the following statements is incorrect?

Diborane $(B_2H_6)$ reacts independently with $O_2$ and $H_2O$ to produce,respectively

Identify the correct statements from the following :
$I$. $Ga_2O_3$ is an amphoteric oxide.
$II$. The dimer of aluminium chloride has three $Al-Cl-Al$ bridge bonds.
$III$. Boron is a very hard refractory solid of high melting temperature.

Pick the correct statement.
$I$. Borax is a white crystalline solid containing $\left[B_4 O_5(OH)_4\right]^{2-}$ units.
$II$. The aqueous solution of borax is acidic in nature.
$III$. Cobalt gives a blue color in the borax bead test.

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