Final answer:
Guanine is an example of a purine, which is a nitrogenous base with a double ring structure found in both DNA and RNA.
Step-by-step explanation:
An example of a purine is B) guanine. In the context of nucleotides and the structure of DNA and RNA, there are specific nitrogenous bases that are categorized as purines or pyrimidines. Purines, such as adenine and guanine, have a distinct double ring structure which incorporates several nitrogen atoms. Conversely, pyrimidines such as cytosine, thymine (in DNA only), and uracil (in RNA only) have a simpler single ring structure. When forming the structure of DNA, these bases pair up with a complementary base, with adenine pairing with thymine and guanine pairing with cytosine, connected by hydrogen bonds to maintain the double helix structure.