Which of the following bases are classified as pyrimidines?

Pyrimidines are one of the two categories of nitrogenous bases found in nucleic acids, the other being purines. Pyrimidines are characterized by their single-ring structure and include three specific bases: cytosine, uracil, and thymine.

In the given answer choice, cytosine, uracil, and thymine are correctly identified as pyrimidines because they all have a six-membered ring structure that includes nitrogen atoms, which is the defining characteristic of this group.

In contrast, adenine and guanine, which appear in other answer choices, are purines and have a double-ring structure. This fundamental distinction between pyrimidines and purines is crucial for understanding nucleic acid structure and function, such as during DNA and RNA synthesis, where the complementary base pairing occurs between these bases.