Final answer:
Based on the given percentages, the large nucleic acid molecule is most likely single-stranded RNA.
Step-by-step explanation:
The large nucleic acid molecule described in the question has 20% A (adenine), 30% C (cytosine), 30% G (guanine), and 20% U (uracil). Based on these ratios, it is most likely b) single-stranded RNA. RNA is typically single-stranded and contains uracil instead of thymine. Additionally, the percentages of the different nitrogenous bases align with a typical RNA molecule. Furthermore, since DNA is double-stranded and has base-pairing rules where adenine pairs with thymine and cytosine pairs with guanine, thus both A and T, or C and G would be present in equal amounts, which isn't the case here. In contrast, RNA is typically single-stranded and can contain varying percentages of its constituent nucleotides, with uracil replacing thymine. Therefore, a nucleic acid that has A, C, G, and U in the percentages given is most likely single-stranded RNA.