The correct answer is A) A the detonation of a bomb on Russian soil.
The detonation of a bomb on Russian soil was the event prompted the United States to take steps to protect itself from Soviet espionage.
The United States intelligence service has closely followed Russia atomic bomb proyect since the detonation of a USSR bomb in the late 1940s, in Russia territory. The U.S. intelligence service paid attention during the cold war years of the uranium and plutonium reserves of Russia.
The U.S. identify early in the 1920s the Russia espionage through many agencies such as the NKVD, OGPU and GRU. That is why until this days, the intelligence service of the U.S. has created complex espionage systems to counter-attack the Soviet’s efforts in espionage.