The correct option is A 2-butene
H3C−O||C−CH2−CH2−CH3Tautomerism⇌H3C−OH|C=CH−CH2−CH3
Keto form enol form
So, a reaction which involves simple proton transfer in an intramolecular fashion is called a tautomerism.
Hence, phenol and lactic acid can also show tautomerism.