Final answer:
The nucleic acid with 20% adenine, 30% cytosine, 20% guanine, and 30% thymine matches the base-pairing rules of DNA and is most likely a double-stranded DNA molecule. The correct option is A. A double-stranded DNA molecule.
Step-by-step explanation:
The nucleic acid molecule described with 20% A (adenine), 30% C (cytosine), 20% G (guanine), and 30% T (thymine) is most likely A. A double-stranded DNA molecule. In the structure of DNA, adenine always pairs with thymine, and cytosine always pairs with guanine.
This complementary base pairing results in equal amounts of A and T, and equal amounts of C and G, which is consistent with the percentages provided in the question. Hence, this pattern of percentages reflects the base-pairing rule in the DNA double helix structure, where A pairs with T and C pairs with G.
The percentages of adenine and thymine are the same, just as the percentages of cytosine and guanine are equal, supporting the idea that the sample is double-stranded DNA, with each strand complementing the other in the helix. Since RNA contains uracil instead of thymine and is typically single-stranded, the given percentages are not characteristic of RNA molecules. The correct option is A. A double-stranded DNA molecule.