During World War II four countries were fully incorporated into the Soviet Union. Latvia, Lithuania and Estonia were three of them. What was the fourth?
Non of the above. They were either parts of other countries, just changed shape, or became part of the Soviet Union at the end of the Russian Civil War in the early twenties. Coventry was never independent - always very dependent. This country can be found in prewar atlases as a separate entity. I have a copy of my Mother's Philip's School Atlas showing it. Like Finland and the Baltic States it was a part of Russia before the October Revolution.
Another less wild shot - The Tuvan People's Republic. Once part of the Tsarist empire - became a Soviet satellite state until it was formally annexed into the U.S.S.R. in 1944
Manchuria is part of China that was ruled by the Japanese. Turkmenistan is an independent now. This ex-country is still part of Russia. It was independent from 1921-1944, though by the time the USSR incorporated it it had become a satellite state in the same way the Eastern Bloc did after the war. It's foreign policy was dictated by Russia after 1929 when the president DonDuc was overthrown in a Moscow inspired coup.