Analysis: Acetic acid reacts with caustic soda to produce sodium acetate, which is an ionic compound with high boiling point and can be easily separated from acetaldehyde by fractionation. After the reaction, sodium acetate reacts with concentrated sulfuric acid to obtain acetic acid again.

When Na2CO3 is added to option D, acetic acid will generate sodium acetate, but both sodium acetate and acetaldehyde are very soluble in water, so it is difficult to separate them by extraction.

Wrong option b: acetaldehyde reacts with silver ammonia solution to generate ammonium acetate, which is difficult to change back to acetaldehyde and does not meet the requirements of separation operation.

Option A: There is no option A in the original answer to this question. The old chalk head thinks this is caused by improper topic setting. In fact, the boiling point of acetaldehyde is about 2 1℃, and the boiling point of acetic acid is about 1 18℃, which is quite different. Therefore, the distillation method is completely feasible under the condition of not too high requirements (and has the advantages of saving reagents and simple operation).