Which of these countries was once part of the Soviet Union?

Estonia was indeed one of the countries that was part of the Soviet Union. It became a constituent republic of the USSR when the Union was established in 1922 and remained so until it regained independence in 1991 following the dissolution of the Soviet Union. This connection to the Soviet Union is significant in understanding Estonia's historical, political, and cultural development.

In contrast, the other countries listed—Romania, Bulgaria, and Hungary—were not part of the Soviet Union. Instead, they were socialist states in Eastern Europe that had their own communist governments, but they remained independent and were not integrated into the USSR. This distinction is important as it illustrates the varying political landscapes of Eastern Europe during the Cold War period.