Final answer:
The U.S. Supreme Court imposed a moratorium on the death penalty in the U.S. in the case of Furman v. Georgia (1972), ruling that the arbitrary and inconsistent imposition of the death penalty violated the Eighth and Fourteenth Amendments.
Step-by-step explanation:
The case in which the U.S. Supreme Court imposed a moratorium on the death penalty in the U.S. was Furman v. Georgia (1972).
In this case, the Supreme Court ruled that the arbitrary and inconsistent imposition of the death penalty violated the Eighth and Fourteenth Amendments, constituting cruel and unusual punishment.
This decision initiated a de facto moratorium on executions that lasted until the Supreme Court's decision in Gregg v. Georgia (1976).