In a DNA molecule, which pair of nitrogen bases are complementary?

In a DNA molecule, adenine and thymine are considered complementary nitrogen bases because they form specific hydrogen bonds with each other. Adenine, a purine base, pairs with thymine, a pyrimidine base, through two hydrogen bonds, which stabilizes the DNA structure. This complementary pairing is essential for the accurate replication and transcription of genetic information, as it ensures that each strand of DNA can serve as a template for the formation of its complementary strand.

Other pairs mentioned, such as adenine and guanine or thymine and cytosine, do not exhibit this complementary relationship. Guanine, another purine like adenine, pairs with cytosine, and in RNA, uracil replaces thymine, pairing with adenine instead. However, since the question specifically asks about a DNA molecule, only the pairing of adenine with thymine is correct in this context.