Which of the following elements has the highest melting point?

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:

Boron is unable to form $BF_{6}^{3-}$ ion. Explain.

Aluminium $(III)$ chloride forms a dimer because ........

Which of the following is the strongest Lewis acid?

Which of the following is the correct decreasing order of Lewis acid strength of boron halides?