The correct answer is C) Soviet Union.
After the meetings between the US, Great Britain, and Soviet Union at the Yalta and Potsdam Conferences, it was clear that the US and Soviet Union had very different plans for a post World War II society. The Soviet Union was looking to expand its communist state while the US was trying to spread capitalism and democracy.
One of the reasons behind Truman's decision to drop the bomb was to ensure that the Soviet Union did not gain influence in the Pacific Ocean. Along with this, Truman wanted to show the Soviet Union what the US military was capable of.