Double-referent check: select all sentences that contain a resumptive-pronoun error.
The contract we signed starts Monday.
The boxes that I labelled them are upstairs.✓Richtige Antwort
The report which arrived today is brief.
The pilot who she landed the aircraft safely thanked the crew.✓Richtige Antwort
Erklärung
Them repeats the boxes already represented by that. In the pilot sentence, the aircraft fills the object role of landed, so who represents the pilot as the relative-clause subject and she wrongly repeats that subject; the contract sentence instead leaves a standard object gap.