Which nitrogenous base pairs with adenine in DNA?

Adenine pairs with thymine in DNA through the formation of two hydrogen bonds. This pairing is a fundamental aspect of the base pairing rules in the DNA double helix structure, where adenine, a purine, is always matched with thymine, a pyrimidine. This complementary base pairing is crucial for the accurate replication of DNA and the preservation of genetic information during cell division.

In the context of the other choices, uracil is a nitrogenous base found in RNA, where it replaces thymine, but it does not participate in DNA base pairing. Cytosine pairs with guanine in DNA, forming three hydrogen bonds, but it does not pair with adenine. Hence, thymine is the appropriate partner for adenine in the structure of DNA. This specificity of base pairing is essential for the proper functioning of genetic processes, including transcription and replication.