Which pyrimidine nitrogenous base pairs with adenine in DNA?

In DNA, the pairing between nitrogenous bases is fundamental to its structure and function. Adenine, a purine, specifically pairs with thymine, which is a pyrimidine. This pairing occurs through the formation of two hydrogen bonds, creating a stable connection that contributes to the overall stability of the DNA double helix structure.

Thymine, having a single six-membered ring structure containing nitrogen, complements adenine's two-ring structure nicely through its unique hydrogen bonding capacity. This specific base pairing is critical for processes such as DNA replication and transcription, ensuring that the genetic information is accurately copied and transmitted.

While options like guanine and cytosine are indeed important in DNA (as they pair with one another), they do not pair with adenine. The other pyrimidine base, cytosine, also does not pair with adenine but instead pairs with guanine. Therefore, thymine is the correct answer because it is the only nitrogenous base that pairs with adenine in the context of DNA.