Key Properties of Thymine in DNA

Steps:

• Thymine is a pyrimidine base with a single-ring structure.
• Pyrimidines like thymine are not purines, which have double-ring structures.
• Thymine base-pairs with adenine via two hydrogen bonds.
• Thymine does not pair with cytosine, which pairs with guanine via three hydrogen bonds.

Why A is correct:

• Option A accurately marks thymine's single ring (✓), non-purine status (✗), two hydrogen bonds with adenine (✓), and lack of pairing with cytosine (✗), matching DNA base-pairing rules.

Why the others are wrong:

• B incorrectly states two hydrogen bonds as false (✗) and pairing with cytosine as true (✓).
• C wrongly assigns double ring (purine ✓) and single ring as false (✗) to thymine.
• D mislabels single ring as false (✗) and purine as true (✓).

Final answer: A