Which of the following has covalent bonds?
A) \[{\rm{N}}{{\rm{a}}_{\rm{2}}}{\rm{S}}\]
B) \[{\rm{AlC}}{{\rm{l}}_{\rm{3}}}\]
C) \[{\rm{NaH}}\]
D) \[{\rm{MgC}}{{\rm{l}}_{\rm{2}}}\]

Hint: A covalent bond is formed due to the share of electrons between atoms. The covalent character defines the measure of an atom’s tendency to attract an electron pair towards itself. Here, we have to identify the covalent character in these ionic compounds.

Complete step by step solution:Let’s discuss the Fazan’s rule that helps to determine the covalent character of these compounds. Fazan’s rule stated that an anion’s covalent nature increases with increase of its polarization. The extent of an atom’s polarization is dependent on two factors namely the cation’s polarizing power and the anion’s polarizability.
Let’s discuss some polarizing properties of cations.
1) When the size of a cation increases, its polarizing property decreases.
2) When the charge of cation increases, its covalent character also increases.
3) Cations having the pseudo noble gas configuration have more covalent character.
Let’s discuss the given ionic compound. So, we have to observe the cations. The charges of the cations are \[{\rm{N}}{{\rm{a}}^{\rm{ + }}},{\rm{A}}{{\rm{l}}^{{\rm{3 + }}}},{\rm{M}}{{\rm{g}}^{{\rm{2 + }}}}\] . Among the cations, aluminium has the charge of +3, that is, (\[{\rm{A}}{{\rm{l}}^{{\rm{3 + }}}}\]), more the charge on cation, the size decreases, that causes the increase of the tendency of the cation to polarise the anion. Therefore, it shows the highest covalent character.

Hence, option B is right.

Note: It is to be noted that, when the size of anions increases, its covalent character also increases. Also, the increase of the charge of anion means the increase of the covalent character of the compound.