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In the case of alkali metals, the covalent character decreases in the order
A
MI > MBr > MCl > MF
B
MCl > MI > MBr > MF
C
MF > MCl > MBr > MI
D
MF > MCl > MI > MBr
Detailed Solution
By Fajans' rules, for a given cation the covalent character of an ionic compound increases with the polarisability of the anion.
The larger the anion, the more loosely its outer electrons are held and the more easily it is polarised (distorted) by the cation.
Size of halide ions: $F^- < Cl^- < Br^- < I^-$
So polarisability and covalent character increase in the order MF < MCl < MBr < MI.
Decreasing order of covalent character: MI > MBr > MCl > MF
The larger the anion, the more loosely its outer electrons are held and the more easily it is polarised (distorted) by the cation.
Size of halide ions: $F^- < Cl^- < Br^- < I^-$
So polarisability and covalent character increase in the order MF < MCl < MBr < MI.
Decreasing order of covalent character: MI > MBr > MCl > MF