Guanine is a purine, not a pyrimidine. The purines are guanine and adenine, while the pyrimidines are cytosine, thymine, and uracil. Uracil is present only in RNA, and thymine only in DNA. The rest of the bases are present in both DNA and RNA.

It is true that DNA is more stable than RNA. While the exact chemical reasons for this are complex, it is useful to know RNA is readily hydrolyzed in basic conditions. In order to make sense of this, remember that DNA acts as the genetic code, and must hold that genetic information for relatively long periods of time. While there are several types of RNA, it typically acts as a messenger, and is degraded after completing its task. While DNA does contain the base thymine and RNA does contain uracil, this is unrelated to the relative stabilities of the two nucleic acids. 

While it could be useful to know that the bases are at the core of the DNA while the sugar-phosphate chains are on the outside, it is possible to answer the question without having memorized that fact. The bases (thymine, cytosine, guanine, adenine) are non-polar, and will cluster in the middle of the chain, away from water. They also connect to each other via hydrogen bonding. On the other hand, the sugar-phosphate groups are hydrophilic, and will cluster towards the outside of the molecule.

All DNA contains 50% purines and 50% pyrimidines due to Watson-Crick base pairing.

However, doing the calculations based on the information given can also give the correct answer. Adenine (A) and guanine (G) are purines. Cytosine (C) and thymine (T) are pyrimidines. Chargaff's rules state that in DNA, G = C and A = T. If the sample is 15% G, then it must also be 15% C. This leaves 70% for A and T, or 35% each. 15% C +  35% T = 50% pyrimidines. 

Chargaff's rules state that guanine (G) = cytosine (C) and adenine (A) = thymine (T). Therefore, since the sample contains 35% C, it must also contain 35% G. 100% - 70% (G + C) leaves 30% left for A and T, or 15% T.

The lagging strand is made up of Okazaki fragments due to discontinuous replication. Each of the fragments has its own primer made from RNA that needs to be removed and replaced with dNTPs. The exonuclease performs this function. A ligase comes through immediately after the exonuclease and joins the fragments together. Helicase is the enzyme that separates the DNA strands.

Hydrogen bonds contribute solely to the base specificity of nucleic acids. Guanine and cytosine share three hydrogen bonds, while adenine and thymine share two. Incorrect base pairing will not have very favorable binding because hydrogen atoms and eletronegative atoms aren't lined up properly. The stability of DNA comes from favorable stacking interactions and hydrostatic effects of the hydrophobic bases and hydrophylic backbone.

The correct answer is "DNA is always double-stranded and RNA is always single-stranded." While this is usually true, there are viruses that have single-stranded DNA and double-stranded RNA. All other answers are true. There is no thymine in RNA, so adenine binds to uracil instead. The ribose sugar on RNA has a hydroxide group not found on the deoxyribose sugar of DNA. This extra group makes RNA more reactive and less stable than DNA. RNA is frequently used by cells for its enzymatic activity in the form of ribosomes, while DNA is used only as a storage of information.

Galactose and glucose are both 6-carbon monosaccharides. They are classified as structural isomers. This means that they have the same molecular formula, but different structural orientations. The molecular formula of a generic monosaccharide with  carbons is ; therefore, the molecular formula for both glucose and galactose is . They have the same number of hydrogen and oxygen atoms.

The chemical structures of glucose and galactose differ in the C4 atom. The hydroxyl group is oriented differently at this position, altering the stereochemistry at C4. All other carbon atoms in glucose and galactose have the same stereochemistry. This means that glucose and galactose are a special type of structural isomer called epimers.

Sucrose is a disaccharide that is made up of a glucose and a fructose molecule, bound by a glycosidic linkage. A disaccahridase, called sucrase, breaks down sucrose molecules into their component monosaccharides (glucose and fructose), which can then by absorbed by the enterocytes in the small intestine.

Fructose is a monosaccharide that can be directly absorbed by enterocytes. Starch is a complex carbohydrate (polysaccharide) with many glucose molecules attached via glycosidic bonds.