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

In a DNA molecule, the nitrogen bases are pairs of complementary bases that bond with each other through hydrogen bonds. Adenine (A) pairs specifically with Thymine (T) in DNA, forming a stable structure essential for the double helix formation of the DNA molecule. This pairing is due to the specific chemical structure of these bases, where adenine forms two hydrogen bonds with thymine.

In the context of DNA, the other options do not represent complementary base pairs. Guanine and Adenine are both purines, and they do not pair with each other. Similarly, Thymine and Cytosine do not bond together; Thymine (a pyrimidine) pairs with Adenine, while Cytosine pairs with Guanine in the DNA structure. The pairing of Guanine and Uracil is relevant in RNA rather than DNA, as Uracil replaces Thymine in RNA molecules, indicating a different base pairing structure in that context. Thus, the pairing of Adenine with Thymine is the clear and correct example of complementary bases in DNA.