$AlC{l}_3\text{is covalent while}Al{F}_3$ is ionic. This can be justified on the basis of:

The correct option is B Fajan's rule
Fajan's Rule: It says covalency (covalent character) is introduced in a bond due to the polarization of the anion by the cation. Factors affecting this polarization are:
1. Cation's size (smaller the size, more is the polarization)
2. Anion's size (greater the size of the anion, more is its polarizability)
3. More the charge on the cation and the anion, more is the covalency introduced in the bond.
Considering these factors, $AlC{l}_3$ can form a better covalent bond because of the larger size of the anion.