The Cold War began
a. when the United States dropped it's first atomic bomb on the Soviet Union. b. when the Soviet Union attacked American troops in China. c. at the conclusion of the WW2 Potsdam Conference. d. as each one of the above took place.
The best answer of the ones given here is C. None of the others is plausible.
At the Potsdam Conference, the Soviet Union and the Western Allies started to divide up Europe and, in particular, Germany. When the did this, they created the basis for the Cold War. They created a situation in which the Soviet Union had control of part of Europe and the Western Allies had control of the other part. Once Europe was divided up in this way, the two sides became more and more suspicious of one another. In this way, the Potsdam Conference can be seen as the start of the Cold War.
In additon, the US never dropped an atomic bomb on Russia and there was no major conflict between the US and the USSR in China.