Final answer:
Uracil is the nucleotide that indicates the nucleic acid is RNA, as it replaces thymine which is found in DNA.
Step-by-step explanation:
The nucleotide that indicates the nucleic acid is RNA is uracil. Both DNA and RNA contain the nitrogenous bases adenine, guanine, and cytosine. However, only RNA contains uracil, which replaces thymine that is found exclusively in DNA. In RNA, uracil pairs with adenine, and this pairing is a defining characteristic distinct from the adenine-thymine pairing seen in DNA. Therefore, seeing uracil in a nucleic acid structure is indicative that the molecule is RNA, distinguishing it from DNA.