If you isolate a single nucleotide from a nucleic acid and determine that the nitrogenous ring structure is cytosine (c), you could say with certainty that the nucleotide could have come

DNA or RNA

Explanation:

Nucleotides are typically made up of phosphate group, nitrogenous bases and pentose sugars. They represent the building blocks of nucleic acids.

The nitogenous bases are of two types - purine and pyrimidine. Purine consists of adenine and guanine while pyrimidine consists of cytosine and thymine/uracil.

Both DNA and RNA consist of cytosine, adenine and guanine. However, thymine is only present in DNA and replaced by uracil in RNA.

Hence, the cytosine could have come from either of DNA or RNA.

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