$\mathrm{AlCl}_3$ is an electron deficient compound but $\mathrm{AlF}_3$ is not. This is because

• A

  atomic size of $\mathrm{F}$ is smaller than $\mathrm{Cl}$ which makes $\mathrm{AlF}_3$ more covalent
• Correct answer
  B

  $\mathrm{AlCl}_3$ is a covalent compound while $\mathrm{AlF}_3$ is an ionic compound
• C

  $\mathrm{AlCl}_3$ exists as dimer but $\mathrm{AlF}_3$ does not
• D

  $\mathrm{Al}$ in $\mathrm{AlCl}_3$ is in $s p^3$ hybrid state but $\mathrm{Al}$ in $\mathrm{AlF}_3$ is in $s p^2$ hybrid state

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