The correct answer is C.

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By extracting with sodium hydroxide, benzoic acid will be converted to its sodium salt, sodium benzoate. Sodium benzoate, unlike its acid counterpart, will dissolve in an aqueous solution. The aqueous layer simply has to be acidified to retrieve benzoic acid after separation. Choice A is wrong because diethyl ether and tetrahydrofuran are miscible; remember, like dissolves like. Hydrochloric acid will not transform benzoic acid into a soluble salt, so choice B is incorrect. Finally, choice D is wrong because benzoic acid is insoluble in water.