Answer is: D. sodium and sulfur.
When the electronegativity difference is greater, the bond polarity is increasing..
Electronegativity (χ) is a property that describes the tendency of an atom to attract a shared pair of electrons.
Atoms with higher electronegativity attracts more electrons towards it, electrons are closer to that atom.
Δχ(Na-S) = 2.6 - 0.95 = 1.655; electronegativity difference between sodium and sulfur.
Ionic bond is the electrostatic attraction between oppositely charged ions (cations and anions).
Sodium is metal (forms cation) and sulfur is nonmetal (forms anion).