Which of the following structures represents guanine?

Step-by-step solution

Guanine is a purine base, which means it has a bicyclic structure consisting of a fused pyrimidine and imidazole ring. This eliminates the single-ring structures (A) and (C), which are pyrimidines (thymine and cytosine, respectively). Among the purines, adenine has an amino group ( -NH₂ ) at the C6 position, which corresponds to structure (B). Guanine has a carbonyl group ( =O ) at the C6 position and an amino group ( -NH₂ ) at the C2 position. Structure (D) correctly matches this description. Answer: