 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 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.