Bonds between A-T (adenine and thymine) are held together by 2 hydrogen bonds, while G-C (guanine and cytosine) are held together by 3 hydrogen bonds. Therefore, A-T bonds are weaker, and will separate first when exposed to heat stress. DNA does not contain uracil.

Guanine always binds to cytosine in DNA and RNA. Thymine always bonds to adenine in DNA. Uracil replaces thymine in RNA, and uracil bonds to adenine.

Adenine and guanine are the purines that bond to the pyrimidines. Cytosine and thymine are the pyrimidines. The purines consist of two carbon rings, and the pyrimidines consist of one carbon ring.

DNA is the hereditary material found in virtually all organisms; however, some viruses use RNA. DNA consists of several components. It has a phosphate-sugar (deoxyribose) backbone and is composed of two strands made from purine-pyrimidine hydrogen bonds in a double helix confirmation. The purines associated with DNA include adenine and guanine and the pyrimidines include cytosine and thymine. Adenine bonds with thymine and cytosine bonds with guanine. 

Each base pair (adenine - thymine and cytosine - guanine) are held together with hydrogen bonds. These bases form the 'ladder steps' part of a DNA molecule. Hydrogen bonds are much weaker than the covalent bonds that hold all the other parts of the molecule together, so when DNA needs to be replicated, its easy for an enzyme to 'unzip' the DNA molecule and expose those bases for replication.

Guanine - cytosine and adenine - thymine form "complimentary base pairs." Guanine can only form hydrogen bonds with cytosine and adenine can only form hydrogen bonds with thymine (and vice versa). With that in mind, any base pairing other than those two can be excluded from this answer. Furthermore, cytosine and guanine form a total of three hydrogen bonds together while adenine and thymine only form two. The extra bond between guanine and cytosine makes the pairing about 50% stronger.

Purines are one of the two families of nitrogenous bases, the other being pyrimidines. Purines consist of a double ring structure, while pyrimidines contain only a single ring making them smaller than purines. Adenine and guanine are purines, while their complimentary base pairs (thymine and cytosine) are pyrimidines. If two purines were to pair together, there would be an unstable bulge in the DNA due to a purine-purine pair being slightly larger than purine-pyrimidine complimentary base pairs. If two pyrimidines were to create a pair, there would be a slight pinch in the DNA for the same reasons. The backbone of DNA consists of deoxyribose and phosphate, not the nitrogenous bases. 

For DNA: A nucleotide contains a nitrogenous base (adenine, thymine, cytosine, or guanine), a pentose (five-carbon) sugar (in this case, deoxyribose), and a phosphate group. A nucleoside is simply a nitrogenous base and a sugar. You could also say that a nucleotide is a nucleoside with an attached phosphate group. 

We will assume this is the "perfect" DNA molecule and there are no anomalies or mutations.

That being said, we know that the amount of adenine (A) is equal to the amount of thymine (T), and the amount of guanine (G) is equal to the amount of cytosine (C) due to the complementarity of DNA; A=T and C=G. We are dealing with percentages, so the total amount of each 4 nucleotides will equal 100.

The amount for adenine is given; A = 23%. Because A=T, the amount of thymine must be 23% as well. Add these two values together to get the total amount of A and T : 23 + 23= 46. 

Again, since we are dealing with percents, we subtract the total amount of A and T from 100: 100 - 46 = 54. Now we know the total amount of C and G is 54. Since the amount of C=G, we can divide 54 by 2 = 27.

The amount of Cytosine in the molecule is 27%, and the amount of Guanine is 27%.

Adenine and Guanine are purines, therefore that answer choice is correct. Thymine and Cytosine are pyrimidines, therefore any answer that involves those choices are incorrect.