Answer: D. It established the idea of a reasonable expectation of privacy.
Step-by-step explanation:
In Katz v. United States, the FBI had arrested Charles Katz who was into illegal sports betting after listening in on his conversation in a phone booth without a search warrant. They listened to his conversation from outside the phone booth because they did not want to break the Fourth Amendment which demanded that they use a warrant.
The lower courts supported the FBI until the case reached the Supreme Court which stated that the FBI was in breach of the Fourth Amendment because of the idea of a reasonable expectation of privacy which meant that even if a person was in the public domain, there were things that they wanted to keep private and the Fourth Amendment protected this.