The correct answer is C) Sector search. A sector search is the appropriate type of search to conduct when a person has fallen overboard and is no longer sighted. This search pattern involves dividing the search area into sectors and systematically searching each sector to thoroughly cover the area where the person is most likely to be located. This is the preferred search method when the person's exact location is unknown, as it allows the search area to be methodically covered. The other options are not as suitable in this scenario. A datum-drift search is used when the exact location of the person is known, a parallel track pattern is more appropriate for searching larger areas, and an expanding circle search is typically used when the person's location is not known but the initial point of entry into the water is.