Electronegativity is defined as the tendency of an atom to attract an electron in a bond that it shares with another atom. Because oxygen wants to receive two elctrons, while both sodiums wish to lose one electron, oxygen has a higher electronegativity than sodium. Typically, electronegativity can be seen as increasing as you go to the top right of the periodic table. For example, fluorine has a higher electronegativity than nitrogen.

Ionic bonds most commonly form between metals and non-metals. Non-metals have high electronegativities and metals have low electronegativities. In these type of bonds, electrons are transferred from the metal to the non-metal because of the large difference in electronegativities. The non-metal will try to gain an electron (high electronegativity) and the metal will try to donate an electron (low electronegativity). The result is a complete electron transfer, known as an ionic bond.

Electronegativity is the tendency of an atom to attract an electron. Elements with very high electronegativities are found at the upper right of the periodic table, while those with low electronegativities are found at the lower left. When two atoms have a very large difference in electronegativity, one atom will have a much greater tendency to attract an electron than the other. As a result, the more electronegative element will pull an electron away from the less electronegative element, creating an electron transfer. This transfer is an ionic bond.

Most non-metals have very high electronegativities, while most metals have very low electronegativities. This is why ionic bonds usually form between a metal atom and a non-metal atom.

There are two primary types of intramolecular bonds: ionic bonds and covalent bonds. In an ionic bond, an electron is transferred (donated) from one atom to the other, usually allowing both atoms to satisfy the octet rule. In a covalent bond, electrons are shared between two atoms in order to allow both to satisfy the octet rule.

Ionic bonds usually form between a metal and a non-metal. HCl, NaCl, and NaOH are all examples of molecules that contain ionic bonds.

Ionic bonds occur when there is a large electronegativity difference between two atoms, such as elemental between fluorine and potassium. The transfer of one electron will fill the valence electron shell of both atoms. Remember that ionic bonds result in ions, or charged species. A covalent bond is a chemical bond in which two atoms share one or more electrons, but the electron(s) is/are never fully donated or accepted.

An ionic bond is a complete transfer of electrons between atoms which forms a cation and an anion. A metal (cation) tends to gain electron(s) and a non-metal tends to lose electron(s). Both act to satisfy their octets and the electrostatic attraction between the oppositely charged atoms holds them together.

Diatomic chlorine forms covalent bonds since they are equal in terms of electronegativities.

 forms a polar covalent bond based on the two atom's differing electronegativities, and on the fact that both atoms are nonmetals.

 has both ionic bonds (between the metal  and polyatomic ion) and covalent bonds within the polyatomic ion. Since we are only looking for ionic bonds,  is the correct answer.