Final answer:
The correct answer is that thymine would be 30 percent of the base composition in the DNA sample. Chargaff's rule dictates that the amount of guanine equals the amount of cytosine, and the amount of adenine equals the amount of thymine, which means that since the sample has 20 percent guanine, it must also have 30 percent thymine.
Step-by-step explanation:
The question pertains to the complementary base pairing nature of DNA, according to Chargaff's rule. If the sample of DNA has 20 percent guanine, then according to the rule, it must have 20 percent cytosine as well, because guanine (G) always pairs with cytosine (C). Since DNA is composed of four bases, and the sum of their percentages must equal 100 percent, the remaining 60 percent must be divided equally between adenine (A) and thymine (T), because adenine always pairs with thymine. Therefore, if guanine and cytosine each make up 20 percent of the bases, adenine and thymine must each compose 30 percent of the bases. This leads us to the conclusion that the correct statement among the provided options is B) Thymine is 30 percent.
Given that the DNA sample has 20 percent guanine, the correct statement is B) Thymine is 30 percent. This follows from Chargaff's rule of base pairing, where adenine pairs with thymine, and guanine with cytosine.