What was the main purpose of the US reply?
o to apologize to the USSR
o to cast the blame on the USSR
O to rescue Powers

I hope there was more information on the question but I'll try to answer according to how I understand it.

The answer is: to cast a blame on the USSR

Explanation:

I believe the question is related to the "Cold War." This war happened after Germany surrendered to the USA.

Before the war, the USA and the Soviet (USSR) were already allies against the "Axis Power" (Japan, Germany and Italy). However, it was said that the Soviet already resented the USA especially at one point when it couldn't help the Russians earlier during the World War II. The USA, also has been very cautious about the Russian's "communism" even before the events. So, both of these countries have already been wary of each other.

The main purpose of the US reply was to cast the blame on the USSR. The reply was made in a telegram by George Kennan, a diplomat. It talked about the "containment strategy." This means that the USA could block the Soviet any time they feel like they're a risk to the nation. This even resulted to an advancement in the Cold War, such as the two countries empowering on "atomic bombs."