Boron halides are usually Lewis acids in nature. This is due to:

Boron cannot form which one of the following anions?

White fumes appear around the bottle of anhydrous aluminium chloride. Give reason.

Which of the following does not exist in free form?

Which of the following is an incorrect statement about the compounds of group $13$ elements?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: Boron is unable to form $BF_{6}^{3-}$.
Reason $R$: Size of $B$ is very small.
In the light of the above statements,choose the correct answer from the options given below.