These are nucleotides that have a single ring structure. example: cytosine, thymine, and uracil

The question is incomplete.

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"Nucleotides with a single ring structure"

I tackled that part, also providing explanation to the point you focused on.

Explanation:

Nucleotides are compounds in which nitrogenous bases (purines and pyrimidines) are conjugated to the pentose sugars (ribose or deoxyribose) and at least one phosphate group. Thus a nucleotide consists of a nitrogenous base, pentose sugar and at least one phosphate group.

Examples of the nitrogenous bases are Adenine, Guanine, Thymine, Uracil and Cytosine. Of all, Thymine, Uracil and Cytosine are with single ring, while Adenine and Guanine are double ring structure.

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the answer will be a) proteins i believe