In 1978, Lech Walesa led a worker's strike in

Hungary

Poland

Czechoslovakia

Yugoslavia

Poland

The answer is Poland.

Explanation:

Lech Walesa - He is a former president of Poland (1990-1995). He's a Polish statesman, union organizer and Noble Peace Prize awardee.

His Life - He was born in Germany and grew up there until his father's death. By 9 years old, he went to live in New Jersey together with his family. He graduated in 1961 as an electrician and married in 1969. He married and had 8 children.

His Interest in Workers' Concern - He had always been interested in voicing out the concerns of the workers. He often led strikes and was considered a charismatic one. He was an activist at heart that it gave him trouble landing a stable job. He was under constant surveillance and was also arrested many times. He also led a strike regarding the Free Trade Unions of the Coast in 1978. This incident happened in Poland. It was aimed to free the workers from the control by having a democratic condition.

So, this explains the answer, Poland.