Document the collection and require a sign-off when possession changes is the correct option to maintain chain of custody when collecting a mobile device during an investigation. It is important to document the collection process, including who collected the device, when and where it was collected, and any other relevant details. It is also necessary to require a sign-off when possession changes, to ensure accountability and track the device's movement. This helps maintain the integrity of the evidence and ensures that it can be used in legal proceedings.