According to the Supreme Court in U.S.v.Montoya de Hernandez,involving a detention at the border for drug investigation:

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In U.S. v. Montoya de Hernandez, the Supreme Court held that the Fourth Amendment's balance of reasonableness is qualitatively different at the international border than in the interior of the country. The Court recognized that the government's interest in preventing the entry of illegal drugs and other contraband is at its highest at the border, and therefore, a brief detention and questioning based on factors that are not in themselves indicative of criminal activity may be justified. However, if the duration or nature of the detention exceeds what is reasonably necessary to effectuate the purpose of the stop or if it becomes a full-blown search, then the intrusion must be justified by reasonable suspicion or probable cause. Therefore, option D is the correct choice.