The covalency and oxidation state respectively of boron in $[BF_4]^-$ are

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

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:

Which of the following boron compounds is the hardest?

The total number of acidic oxides from the following list is: $NO$,$N_2O$,$B_2O_3$,$N_2O_5$,$CO$,$SO_3$,$P_4O_{10}$

In which of the following,a salt of the type $KMO_2$ is obtained?