Answer:
a. Czechoslovakia
Explanation:
"On August 20, 1968, the Soviet Union led Warsaw Pact troops in an invasion of Czechoslovakia to crack down on reformist trends in Prague. Although the Soviet Union's action successfully halted the pace of reform in Czechoslovakia, it had unintended consequences for the unity of the communist bloc."
more detailed info found here; https://history.state.gov/milestones/1961-1968/soviet-invasion-czechoslavkia
