Which of the following nitrogenous bases is a pyrimidine?

Thymine is identified as a pyrimidine nitrogenous base, which is a classification referring to its structure. Pyrimidines are characterized by a single six-membered ring containing nitrogen, whereas purines, like adenine and guanine, consist of a double-ring structure formed from a six-membered and a five-membered ring.

Thymine is one of the primary pyrimidines found in nucleic acids, specifically in DNA, where it pairs with adenine. It differs from purines, both structurally and in its function within the genome. Understanding the distinction between pyrimidines and purines is fundamental in genetics, as it relates to DNA and RNA structure, replication, and the synthesis of nucleic acids.

Arginine, while it contains nitrogen and is involved in various biological processes, is actually an amino acid and not a nitrogenous base, which further clarifies why thymine is the appropriate choice in this context.