Which folate structure (from the list below) (a) Is the substrate for the enzyme that is inhibited

Question:

Which folate structure (from the list below)
(a) Is the substrate for the enzyme that is inhibited by methotrexate and trimethoprim?
(b) Has the most highly oxidized one-carbon substituent?
(c) Is used in the conversion of serine to glycine?
(d) Transfers its one-carbon substituent to a B 12 coenzyme? What amino acid is synthesized as the end result of this reaction?
(e) Is the coenzyme for the thymidylate synthase reaction?
(f) Is not known to exist in nature?
(g) Is used in purine nucleotide synthesis?