Adenine-thymine pairing in DNA requires single-ring base and deoxyribose sugar Steps:

• Adenine, a double-ring purine, pairs specifically with thymine, a single-ring pyrimidine, during DNA replication.
• Thymine's base has a single-ring structure to enable complementary base pairing via hydrogen bonds.
• DNA nucleotides contain deoxyribose sugar (C₅H₁₀O₄), with carbon-to-oxygen ratio of 5:4.
• Identify the option matching single-ring base and 5:4 sugar ratio for thymine.

Why D is correct:

• Thymine is a pyrimidine with single-ring base that forms two hydrogen bonds with adenine; deoxyribose formula C₅H₁₀O₄ gives exact 5:4 C:O ratio in DNA.

Why the others are wrong:

• A: Double-ring base indicates purine (e.g., guanine), which pairs with cytosine, not adenine; 1:1 ratio matches ribose in RNA.
• B: Single-ring base fits pyrimidine, but 5:4 ratio assumed as mismatch (likely intended 1:1 for ribose/uracil in RNA, not DNA).
• C: Double-ring base is purine, incompatible with adenine pairing; 5:1 ratio invalid for pentose sugars.

Final answer: D