Emphatic stress is the placement of greater stress on a particular word or phrase in an utterance for the sake of emphasis or clarification. For questions on emphatic stress, the goal is to look for the question that contradicts the stressed word or phrase in the utterance. The utterance, in this context, is trying to clarify that the mother "can" actually cook the meal. So, it should be the response to the question "Isn't your mother able to cook the meal?"