The melting point of $AlF_3$ is $1040 \, ^{\circ}C$ and that of $SiF_4$ is $-77 \, ^{\circ}C$ (it sublimes) because

• A
  there is a very large difference in the ionic character of the $Al-F$ and $Si-F$ bond
• B
  in $AlF_3$,$Al^{3+}$ interacts very strongly with the neighbouring $F^{-}$ ions to give a three-dimensional structure,but in $SiF_4$ no such interaction is possible
• C
  The silicon ion in the tetrahedral $SiF_4$ molecule is not shielded effectively from the fluoride ions,whereas in $AlF_3$,the $Al^{3+}$ ion is shielded on all sides
• D
  the attractive forces between the $SiF_4$ molecules are strong,whereas those between the $AlF_3$ molecules are weak