Which among the following halogens combines readily with metals to form metal halides with highest ionic character?

Ionic character of metal halides decreases in the order $\mathrm{MF}>\mathrm{MCl}>\mathrm{MBr}>\mathrm{MI}$, where, $\mathrm{M}$ is a monovalent metal. Smaller the size of anion, greater is the ionic character (Fajan's rule). Therefore, fluorine forms metal halides with highest ionic character.