= Solution
<Hydroxide> attack at the aromatic carbon bearing the leaving-group oxygen, followed by <SNAr reaction>, would put solvent-derived $^{18}$O into the liberated dinitrophenol. Its absence rules out that aryl C-O cleavage route, conventionally called \b[BAr2], as the significant pathway.
Acyl C-O cleavage instead releases the original aryloxy oxygen with the leaving group, so unlabeled dinitrophenol is expected for both tetrahedral acyl substitution and the elimination route below.
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