When did the America became aware of the weapons placed in Cuba by USSR ?

Three week later

The correct answer is option (3) - Three week later

In April 1961, the leaders of the Union of Soviet Socialist Republics (USSR) were worried that the United States of America (USA) would invade communist-ruled Cuba and overthrow Fidel Castro, the president of the small island nation off the coast of the United States. Cuba was an ally of the Soviet Union and received both diplomatic and financial aid from it. Nikita Khrushchev, the leader of the Soviet Union, decided to convert Cuba into a Russian base. In 1962, he placed nuclear missiles in Cuba. The installation of these weapons put the US, for the first time, under fire from close range and nearly doubled the number of bases or cities in the American mainland which could be threatened by the USSR.

Three weeks after the Soviet Union had placed the nuclear weapons in Cuba, the Americans became aware of it. The US President, John F. Kennedy, and his advisers were reluctant to do anything that might lead to full-scale nuclear war between the two countries, but they were determined to get Khrushchev to remove the missiles and nuclear weapons from Cuba. Kennedy ordered American warships to intercept any Soviet ships heading to Cuba as a way of warning the USSR of his seriousness. A clash seemed imminent in what came to be known as the Cuban Missile Crisis.