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

Which of the following reactions will yield the crystalline variety of boron?

Which of the following does not exist in free form?

The correct Lewis acidity order of $BX_3$ is:

$A$ Lewis acid '$X$' reacts with $LiAlH_{4}$ in ether medium to give a highly toxic gas. This gas when heated with $NH_{3}$ gives a compound commonly known as inorganic benzene. The gas is

Aluminium reacts with $NaOH$ and forms compound '$X$'. If the coordination number of aluminium in '$X$' is $6$, the correct formula of $X$ is: