Which of the following DOES NOT contribute to the ring of thymine ?
The question asked specifically about the ring and not side groups present on ring. In Thymine (Pyrimidine) ring, sources of N & C are: N3 from Glutamine C2 from CO2 (or Bicarbonate) N1, C4, C5, C6 from Aspaate -CH3 is not pa of the ring but a side group which is contributed by THF, Hence it is the answer.
Aspaate
Glutamine
THF
Bicarbonate
Nucleotides