Which of these countries was never a member of the Warsaw Pact?

Yugoslavia were never a member of the Warsaw Pact.

The Warsaw Pact, formally the Treaty of Friendship, Co-operation, and Mutual Assistance, was a collective defence treaty among the Soviet Union and seven Soviet satellite states in Central and Eastern Europe during the Cold War. The Warsaw Pact was the military complement to the Council for Mutual Economic Assistance (CoMEcon), the regional economic organization for the communist states of Central and Eastern Europe. The Warsaw Pact was created in reaction to the integration of West Germany into NATO in 1955 per the London and Paris Conferences of 1954, but it is also considered to have been motivated by Soviet desires to maintain control over military forces in Central and Eastern Europe.

The original members were:

Albania (withdrew in 1968)

Bulgaria

Czechoslovakia

East Germany (withdrew in 1990)

Hungary

Poland (withdrew in 1990)

Romania

Soviet Union