Final answer:
The nucleic acid molecule with 20% A, 30% C, 30% G, and 20% U is most likely a single-stranded RNA molecule, as it contains uracil and does not have equal amounts of adenine and thymine, which would be indicative of double-stranded DNA.
Step-by-step explanation:
If a large nucleic acid molecule has 20% adenine (A), 30% cytosine (C), 30% guanine (G), and 20% uracil (U), it is most likely a single-stranded RNA molecule. Unlike DNA, RNA contains uracil instead of thymine (T) and is generally single-stranded. DNA, on the other hand, would have thymine in place of uracil and is typically double-stranded with adenine pairing with thymine and cytosine pairing with guanine, thus, the percentages of A and T, and C and G, would be the same in a double-stranded DNA molecule. Since the molecule in question has uracil and an uneven distribution between the 'A' and 'G' content, it indicates that the molecule is RNA rather than DNA.