Which one of the following has the highest ionic character?

Ionic character increases when the electronegativity difference between the bonded atoms is large and when the cation is large/low-charge (less polarising) and the anion is also less polarisable, per Fajans' rules. Among the options, K2S involves potassium (a large alkali metal cation with low charge density and very low electronegativity ≈ 0.82) and sulphur (electronegativity ≈ 2.58). The large electronegativity difference combined with low polarising power of K+ gives K2S the highest ionic character. BeF3, SiO2, and NCl3 involve small/highly charged cations or covalent bonds with significant covalent character.