Identify one exception to the general requirement that the authorities obtain a search warrant in a criminal investigation

Answer 1

Exceptions from the text:
a . Consent — A person may freely consent to a search. Another occupant of one's apartment may also consent to a search of shared areas within the dwelling.
b. Incident to a lawful arrest — During a lawful arrest, a defendant is taken into custody. The arresting officer then may make a search, generally limited to an "arm's length" area in which the defendant could reach for a weapon or destructible evidence. Once at the police station, during booking procedures, the warrantless search may take the form of a strip search for weapons or contraband. In many states, a person arrested for minor, misdemeanor offenses (such as unpaid parking violations) could be strip searched at the police station. Because of the unlikelihood that such arrestees might possess weapons or contraband, however, such practices are very rare.
c. Motor-vehicle search based on probable cause — A vehicle may be searched if there is probable cause to believe it contains contraband. Mobility of the vehicle underlies this exception to the rule requiring a search warrant in advance. Note that a traffic offense, such as speeding, may not be used as a pretext to justify a search for evidence of a different crime, but the observation of something "in plain view" is not an unreasonable search. Indeed, it is not considered a search at all and, thus, is not subject to the restrictions of the Fourth Amendment. For example, a handgun lying on the floor behind the front seat, if seen by the officer, would justify a search of a vehicle that had been stopped for a routine traffic offense. The only restriction on the "plain view" doctrine is whether the officer had a right to be in that place when the observation was made.