In 1963, the United States and the Soviet Union signed a treaty stating that

The 1963 Limited Test Ban Treaty prohibited nuclear tests in the atmosphere, underwater, and in outer space, but it did not ban underground tests. So nuclear testing was restricted to underground locations, making “all nuclear testing would be held underground” the best answer. It did not ban the use of nuclear weapons or limit ownership to the United States and the Soviet Union.

AI-written and checked against the verified answer.