Which one of the following molecular hydrides acts as a Lewis acid?

What are the approximate $H-B-H$ bond angles in diborane $(B_2H_6)$?

$A$ compound $(A)$ of boron reacts with $NMe_3$ to give an adduct $(B)$ which on hydrolysis gives a compound $(C)$ and hydrogen gas. Compound $(C)$ is an acid. Identify the compounds $A, B$ and $C$. Give the reactions involved.

Boric acid is polymeric due to

Two elements $X$ and $Y$ react with highly electropositive metals to form binary compounds,which upon hydrolysis yield borane and silane,respectively. What are $X$ and $Y$?

Below are two statements,one labeled as Assertion $(A)$ and the other as Reason $(R):$
Assertion $(A)$: Among group $13$ elements,boron's melting point is unusually high $(2453 \ K)$.
Reason $(R):$ Solid boron has a strong crystalline lattice.
In the context of the above statements,choose the correct answer from the following options: