Final answer:
In the story 'Lamb to the Slaughter,' Mary goes shopping for groceries to establish an alibi for herself.
Step-by-step explanation:
In the story 'Lamb to the Slaughter,' Mary goes shopping for groceries because she wants to create an alibi for herself. She plans to use the groceries as evidence that she was out of the house at the time of her husband's murder. By going shopping, Mary establishes a reason for leaving the house and provides herself with a potential witness in the form of the grocer. This helps her create a believable story and avoid suspicion.