No electrovalent bond is 100% ionic in nature. This is because:

Explanation

Option D is correct.When cations and anions approach each other, polarizability increases only. Polarizability is the ability of an electron cloud to be distorted by an electric field. When cations and anions approach each other, the electron clouds of the two atoms are distorted by the electric field of the oppositely charged atom. This distortion allows for some sharing of electrons between the two atoms. Sharing of electrons takes place to some extent only. Even in the most ionic bonds, there is some degree of sharing of electrons between the cation and anion. This is because the electronegativity difference between the two atoms is never 100%. For example, the electronegativity difference between sodium and chlorine is 2.1, which is considered to be a very large difference. However, this still means that there is some electron density shared between the two atoms. Highest ionic character is present in CsF only. The ionic character of a bond is a measure of the degree to which a bond is ionic. It is calculated using the difference in electronegativity between the two atoms involved in the bond. The higher the difference in electronegativity, the more ionic the bond. CsF has the highest ionic character of any known compound, with an electronegativity difference of 3.2. However, even in CsF, there is still some degree of sharing of electrons between the two atoms.