Assertion : $\mathrm{Al}$ forms $\left[\mathrm{AlF}_6\right]^{3-}$ but $\mathrm{B}$ does not form $\left[\mathrm{BF}_6\right]^{3-}$. Reason : $\mathrm{B}$ does not react with $\mathrm{F}_2$.

• A

  If both assertion and reason are true and reason is the correct explanation of assertion.
• B

  If both assertion and reason are true but reason is not the correct explanation of assertion.
• Correct answer
  C

  If assertion is true but reason is false.
• D

  If both assertion and reason are false.

Hence, the correct answer is option: 'C'.