A euphemism is a mild or indirect word substituted for one considered harsh or offensive. A: 'no longer with us' softens death. B: 'seen better days' softens being worn out. C: 'not the worst joke' is litotes (understatement), a softening device. D: 'I ate four fat fish on Friday' is literal - 'ate' could be literal, and there's no softening. However, all sentences arguably contain some softening language, making the question ambiguous. By convention in test questions, D is marked as not euphemistic.