For a covalent bond to be polar, the two atoms that form the bond must have differing atomic weights.
a. differing numbers of neutrons.
b. differing melting points.
c. differing electronegativities.
d. similar electronegativities.

differing electronegativity

Explanation:

the electronegativity is a chemical property that describes the tendency of an atom to attract a shared pair of electrons towards itself.

in ionic bond one of the 2 atoms attract the electrons from the another and don't share it so the electronegativity must be different to make the attract tendency different.

so,

first, when we speak about bond we should deal with electronegativity not the number of the neutrons or the melting points

second if the two electonegativities are similar then the electrons will be shared as in covalent bonds. but if they are different one will lose the electrons for the another one

explanation:

0.07 g/cm³

1.37 g/cm³