For a unit cell, the corners count as 1/8 of a sphere and atoms completely within the unit cell count as 1.  This gives us 1 yellow and 1 green sphere in the unit cell shown above.

The figure above represents ABCABC packing which is cubic close packing.

The figure above represents ABABA packing which is hexagonal close packing.

The figure above has 8 Zn atoms on the corners (counting as 1/8) and 6 Zn atoms on the faces (counting as ½).  This gives 4 Zn atoms in the unit cell.  The Sulfur atoms are completely within the unit cell and count as 1 each for a total of 4 S atoms.  This gives the formula Zn₄S₄ which reduces down to ZnS.

An interstitial alloy has a smaller atom inserted into the unit cell, a substitutional alloy has another atom substitute for the main element, and a pure metal is only composed of a single atom type.

Ammonium is NH₄₊. It is formed by the association of NH₃ with H+. Thus, NH3 acts as a Lewis base, and H+ is a Lewis acid. This is a coordinate covalent bond, and there is one of these in the molecule. The remaning three bonds are polar covalent (N–H bonds).

Covalent compounds generally form between two nonmetals that will both share electrons to complete their octets. Boron (B) is a metalloid and can act as a metal or nonmetal. In its bonds with fluorine (F), however, the two compounds have relatively similar electronegativities. A lack of difference in electronegativity results in covalent bonds.

Ionic compounds form between metals and nonmetals through a transfer of electrons, Metallic compounds are built from only metals. Polyatomic ions will have a formal charge.

Polarity in a bond results from an uneven sharing of electrons. An extreme example is an ionic bond, in which an electron is almost fully transferred from one atom to another due to the second atom's electron affinity.

In order to generate an uneven pull on shared electrons, the atoms involved in the bond must have significantly differing electronegativities. This will cause one atom to pull electrons closer to its nucleus, away from the other atom involved.

Molecular and electronic geometry can affect the polarity of a compound, but do not directly affect polarity of a single given bond. Atomic radius does not play a significant role in polarity.

In carbon has a single bond to each hydrogen and a double bond to oxygen. Carbon must form four bonds to satisfy the octet rule. Only single bonds can be formed with hydrogen, but oxygen can form double bonds. The carbon will thus form two single bonds and one double bond.

The definition of a coordinate covalent bond is that one of the atoms donates both of the electrons being shared. This corresponds with the correct answer choice.