(C) It has weak electrostatic forces of attraction between its oppositely charged ions.
Compound C is ionic (A loses electrons, B gains electrons). Ionic compounds have strong electrostatic forces, not weak ones.
When A loses electrons and B gains electrons, an ionic compound (C) is formed. Ionic compounds have high melting points (A is correct), are highly soluble in water (B is correct), and conduct electricity in molten state or aqueous solution (D is correct). The defining feature of ionic compounds is strong electrostatic forces of attraction — so option (C) is the incorrect property, making it the right answer.