The correct answer is 'that Mary ate at the' i.e. 'error lies in this part of the sentence.' Key Points If two actions take place in the past, one after the other, the 1st action will be in Past perfect tense, and the 2nd action will be in Simple past tense. In the given sentence, a 'that clause' is used in which 'before' is used as a 'conjunction' because 'she' is a subject and 'died' is a verb. Whenever 'before' is used as a conjunction it is hinting that the main clause should be in the past perfect tense. The action is written with the conjunction 'before' is always → the second action. And the action is written in the main clause is always → the first action. Let's see an example- ​He had left the city before the sun rose. Before the sun rose → The second action. He had left the city → The first action. The correct sentence is: 'Tom told the police that Mary had eaten at the restaurant in question before she died.'