Guanine's molecular structure and base pairing in DNA

Steps:

• Recall guanine is a purine base with a double-ring structure, not single-ring like pyrimidines.
• Confirm purines (adenine, guanine) have fused rings; pyrimidines (cytosine, thymine) have one ring.
• Note guanine forms three hydrogen bonds with cytosine, not thymine (which pairs with adenine via two bonds).
• Match features to options: single ring no, purine yes, three bonds yes, pairs with thymine no.

Why C is correct:

• C accurately captures guanine as a purine (double ring, no single ring) forming three H-bonds, though pairing detail needs verification against standard DNA base pairing rules (G-C).

Why the others are wrong:

• A: Incorrectly states single ring yes and purine no, confusing guanine with pyrimidines.
• B: Not enough information (option details missing).
• D: Falsely claims purine no, misclassifying guanine's structure.

Final answer: C