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ANSWER: E. B&C
Keto-enol tautomerism is the equilibrium between a keto form and an enol form (alcohol) of a chemical. In this scenario, both answer choices B and C are applicable keto-enol tautomers, although the frequency of one form compared to another may be shifted. This topic is common in the organic chemistry section of the OAT that falls under conceptual problems. One may be asked to identify a keto-enol tautomer like this problem, or be given a set of equilibrium reactions and asked to describe the relationship.