According to complementary base pairing rules, which of the following is not a correct pairing?

The pairing of adenine with uracil is a correct pairing in the context of RNA, as adenine indeed pairs with uracil during transcription. However, when considering the broader context of both DNA and RNA, adenine typically pairs with thymine in DNA molecules, where thymine is present instead of uracil.

The other pairs listed—cytosine with guanine, adenine with thymine, and guanine with cytosine—are universally recognized as correct according to the complementary base pairing rules in DNA. In DNA, adenine pairs with thymine via two hydrogen bonds, while cytosine pairs with guanine with three hydrogen bonds, contributing to the stability of the DNA double helix.

Therefore, the distinction is drawn: adenine pairs with uracil in RNA but not in the context of DNA, which makes the statement about adenine pairing with uracil not entirely incorrect but contextually not applicable when considering traditional DNA base pairing rules.