Ribonucleosides, which constitute ribonucleic acid (RNA), consist of four heterocyclic "bases" and d-ribose (a sugar). The four heterocyclic bases are guanine, adenine, cytosine, and uracil.
Guanine and adenine are referred to as purine bases due to their structural similarity to purine. Cytosine and uracil are referred to as pyrimidine bases due to their structural similarity to pyrimidine. Which of these heterocyclic bases has an aromatic ring?