Guanine's structure and base pairing Steps:

• Identify guanine as one of the four DNA bases.
• Recall purines (adenine, guanine) have double-ring structures; pyrimidines have single rings.
• Note guanine pairs with cytosine via three hydrogen bonds.
• Match these traits to the option describing double ring, purine status, and three H-bonds.

Why A is correct:

• Guanine is a purine nucleobase with a fused pyrimidine-imidazole ring system and pairs with cytosine through three hydrogen bonds, per Watson-Crick base pairing rules.

Why the others are wrong:

• B: Guanine has a double ring and forms three H-bonds, contradicting both negations.
• C: Guanine has a double ring and is a purine, making the first two clauses false.
• D: Guanine forms three H-bonds with cytosine, not fewer.

Final answer: A