Which of the following is a requirement for conducting a lawful search?

A lawful search is contingent upon certain legal standards that protect individuals' rights as established by the Fourth Amendment. One of the primary requirements is either valid consent or probable cause. Valid consent involves obtaining permission from the person who has the authority over the area to be searched. On the other hand, probable cause exists when law enforcement has sufficient facts or evidence to believe that a crime has occurred or that evidence of a crime can be found in the location to be searched.

These standards help to maintain a balance between law enforcement duties and the constitutional rights of individuals, ensuring searches are not arbitrary or unjust. In contrast, the other options do not meet the legal threshold necessary for a lawful search. A presence of a crowd does not inherently authorize a search, immediate danger to life relates to exigent circumstances which, while they can justify a search without a warrant, do not serve as a general requirement, and a signed affidavit pertains to obtaining a warrant rather than conducting a search itself.