Which of the following is ionic ?

Among the options given, (a) PbF4 is expected to be primarily an ionic compound based on Fajan's rule.

Lead (Pb) has a relatively large atomic radius, and fluorine (F) has a small atomic radius. The cation Pb²⁺ is large compared to the anion F^-. Moreover, the high charge on the Pb²⁺ ion (compared to the charge on other cations such as Sn²⁺) also increases the ionic character of the Pb-F bond.

On the other hand, SnCl₄, CF₄, and SiF₄ are covalent compounds because the bond between the two atoms in each molecule is primarily covalent, and they do not have large enough differences in electronegativity to form ionic bonds.