Which of the following is NOT a nitrogenous base?

Ribonucleotide is the correct answer because it refers to a molecule that comprises a ribose sugar, a phosphate group, and a nitrogenous base, but it is not itself classified as a nitrogenous base. Nitrogenous bases are specific organic compounds that are fundamental components of nucleotides, which include adenine, cytosine, and thymine. These bases are critical for encoding genetic information and are essential in the structure of DNA and RNA.

In contrast, adenine, cytosine, and thymine are all identified as nitrogenous bases, playing significant roles in the formation of nucleic acids. In summary, while ribonucleotide includes nitrogenous bases, it does not qualify as one, making it the option that does not belong to the group of nitrogenous bases.