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.

Only nitrogen, oxygen, and fluorine can form hydrogen bonds since these three elements are very electronegative. Thus, their partial negative charge (and the presence of a lone pair of electrons) attracts a partially positive hydrogen atom. Carbon, on the other hand, is not very electronegative and thus cannot form hydrogen bonds.

Bond strengths are measured in kilojoules/mole. Hydrogen bonds can have a strength of . Ionic bonds can have a strength of . This makes hydrogen bonds much weaker than ionic bonds.

Uracil is not a base in DNA, so A-U can be ruled out as an answer. Moreover, C-G and A-T are the correct base pairs, so the other combinations can be ruled out as well. C-G requires more energy to break because there are three hydrogen bonds between these bases, while A-T has only two hydrogen bonds holding them together.

The substituents of a chiral center are arranged in a specific manner that cannot be superimposed on its mirror image. An example is a carbon atom with four unique substituent groups.

Oxygen is more electronegative than hydrogen. This results in the electrons to spend more time with the oxygen atom making it negatively chared and the hydrogen atoms to be positively charged. The charged water molecule is then able to form hydrogen bonds with polar molecules. 

Proteins will behave similarly to phospholipids in water; the polar groups will form favorable interactions on the surface with water, while the hydrophobic groups will be in the core and away from the water molecules. Usually, amino acids with non-polar residues will be found in the core of proteins. Tryptophan has a nonpolar side chain, and will thus be found in the core of a protein that is in a aqueous environment. 

In aqueous solutions, lipid molecules are surrounded by a lattice-like ring of water molecules known as a clathrate shell. This locks up previously free water molecules in this state, which is not entropically favored. Though it is unavoidable that some water molecules will have to be robbed of some of their freedom of motion by forming at least one clathrate shell, the ideal scenario thermodynamically is the one in which the fewest water molecules are stuck in the shell as possible. As a result, lipid bubbles in aqueous solutions tend to go from many to one, as this results in the  clathrate shell with the fewest number of water molecules. In the process, many smaller clathrate shells are broken, and many water molecules are freed, thus increasing the entropy of the system. 

Hydrophobic effects require water to occur. The reason that hydrophobic groups tend to group together is that by doing so, the network of water molecules around them stays intact. There are no other special forces at play between hydrophobic groups. It is precisely the non-polar nature of hydrophobic groups that gives them their character; water molecules are polar. Cell membranes have a phospholipid bilayer with internal hydrophobic regions (the lipid tails), holding together the membrane.

This is called the hydrophobic effect. Although initially the water molecules arrange themselves in clathrates and become more ordered, their exclusion of the hydrophobic/nonpolar solute is entropically driven and energetically favorable.