Which one of the following elements forms compounds with pronounced covalent character?

The covalent character of an ionic bond is governed by Fajans' Rules, which state that a smaller cation has a higher polarizing power. Among the Group 1 alkali metals (Li, Na, K, Rb), Lithium (Li) has the smallest atomic and ionic radius. Because of its exceptionally small size, the Li⁺ ion possesses a very high charge density, allowing it to strongly distort the electron cloud of an anion. This high polarization leads to a pronounced covalent character in lithium compounds (such as LiCl or LiI) compared to the compounds of other alkali metals. As we move down the group from Sodium (Na) to Rubidium (Rb), the ionic size increases and the polarizing power decreases, resulting in compounds that are more purely ionic in nature.