"Greater is the polarisation, greater is the covalent character." For anions to get polarised, the size of an anion must be big in size, to get distorted by the cation. Therefore, bigger the size of the anion, greater the polarisability, greater the covalent character. Among the given ionic compound, the iodide anion has the largest size, it undergoes polarisation to larger extent. Hence, CaI2 has the largest covalent character.