Which one of the following molecules contains both ionic and covalent bonds ?

The compound ${\mathrm{K}}_2{\mathrm{SO}}_4$ is an ionic. In this compound, sulphur(S) forms covalent bonds with oxygen atoms and Potassium (K) forms ionic bonds with oxygen atoms. Since potassium is a metal and oxygen is a non metal hence electro negativity difference between K and O should be > 1.7. Hence, ionic bond is formed between K and O .

Note: In the remaining three molecules only non metal atoms are present. Hence, covalent bonds are only observed in those three molecules.