Which of the following is a pyrimidine base?

Cytosine is classified as a pyrimidine base, which is one of the two categories of nitrogenous bases found in nucleic acids (the other being purines). Pyrimidines are characterized by having a single ring structure in their molecular composition. This includes cytosine, thymine, and uracil. In contrast, adenine and guanine fall into the purine category, which contains a double ring structure. Thymine also belongs to the pyrimidine group alongside cytosine, but since cytosine is the answer specified, it highlights the understanding of the classification of nitrogenous bases essential in genetics and cellular biology.