The correct answer is C) The Soviet Union knows the captured plane was a US spy plane.
The message of the paragraph is "The Soviet Union knows the captured plane was a US spy plane.
At this moment the government of the Soviet Union knew that this was a problem that had to be informed to the government of the United States. A military aircraft from the United States had trespassed the space of the USSR and had been shot down over Sverdlovsk. The US then responded that it was not a military plane but a research plane working on water research.
During the Cold War years and Arms Race years, both countries lived in permanent tension for incidents and differences like these.