Hydrogen bonding occurs when a molecule contains a hydrogen atom bonded to fluorine, oxygen, or nitrogen. This hydrogen becomes partially positive in charge, while the attached atom becomes partially negative. This is due to unequal electronegativity causing increased electron density for the more electronegative atom, and therefore a more negative charge. The partially positive charge then forms an electrostatic attraction to another partially negative atom nearby.

In the double-helix structure of DNA, there are 2 hydrogen bonds between adenine and thymine, and 3 hydrogen bonds between cytosine and guanine. Phosphodiester bonds are covalent bonds between the 5-C sugar and phosphate group of nucleotides (and are much stronger than the hydrogen bonds between complementary bases).

Hydrogen bonds are found in primary structure of protein, as well as between the bases in DNA structure. Hydrogen bonds are only found between hydrogens attached to oxygen, nitrogen, or fluorine. They increase the attraction between water molecules, therefore are harder to break in large numbers, causing an increase in boiling point.

In a solution of water, the outside of a folded protein is going to be in direct contact with water. Therefore, polar or ionic attractions are most favored on the outside of a protein in a solution of water since these can form attractions. The only amino acid listed that can form either polar or ionic attractions is glutamic acid.

An example of strong intermolecular bond is a hydrogen bond. A hydrogen bond occurs between a hydrogen atom on a molecule and an either nitrogen, oxygen, or fluorine atom on an adjacent molecule. The question states that there are bonds between molecule A and molecule B. We can assume that these are hydrogen bonds. This means that molecule A can have hydrogen atom and/or nitrogen, oxygen, or fluorine atoms. Similarly, molecule B can also have hydrogen atom and/or nitrogen, oxygen, or fluorine atoms.

Molecule C doesn’t form hydrogen bonds with either of the other two molecules. This means it cannot have hydrogen and/or nitrogen, oxygen, or fluorine atoms. This is because if molecule C had any of these atoms then it would interact with at least one of the other molecules and form hydrogen bonds (because molecule A and molecule B have these other atoms).

Covalent bonds occur between atoms within (or intra) molecules. This means that covalent bonds are intramolecular bonds. Hydrogen bonds, on the other hand, occur between hydrogen atoms of one molecule and either a nitrogen, an oxygen, or a fluorine atom in another molecule. Since hydrogen bonds occur between molecules, they are classified as intermolecular bonds. There are some situations in which hydrogen bonds may be formed intramolecularly, but these are special cases. 

Hydrogen bonds occur between a hydrogen atom in one molecule and either nitrogen, oxygen, or fluorine atom in another molecule. It only involves nitrogen, oxygen, or fluorine atoms because these atoms have high electronegativity. Since they have very high electronegativity, these three atoms can easily attract the electron found in the hydrogen atom and form hydrogen bonds. Chlorine doesn’t have high enough electronegativity to form hydrogen bonds.

An atom's state of matter does not determine whether an atom can form hydrogen bonds. Fluorine, just like chlorine, is a halogen and it can form hydrogen bonds.

Hydrogen bonds can occur between two molecules (intermolecular) and within parts of a single molecule (intramolecular). They are weaker than covalent bonds but are stronger than a Van der Waals interaction, and this strength is determined by a number of factors including bond angle. 

For a hydrogen bond to occur, the atom to which the hydrogen is bonded has to have a high relative Pauling electronegativity. Only oxygen, nitrogen, and fluorine, in the upper right corner of the periodic table, have these electronegativities. Neon, argon, and xenon are, of course, inert noble gases, although xenon is sometimes assigned a high electronegativity. This is due, however, to rare bonding events -- certainly not regular hydrogen bonding. Sodium, magnesium and aluminum often form positively charged ions, so they would not tend to attract a hydrogen nucleus.

Hydrogen can form hydrogen bonds with nitrogen, oxygen, and fluorine. A water molecule consists of an oxygen atom bonded to two hydrogen atoms. Each hydrogen atom can form a hydrogen bond with a nitrogen, fluorine, or oxygen atom. Also, the oxygen, which has two lone pairs of electrons, can form two hydrogen bonds with hydrogen atoms. This sums to four hydrogen bonds per water molecule.