Among the countries listed, Germany is often recognized for having long-term success in adopting democracy, especially after World War II with the establishment of the Federal Republic of Germany (West Germany) and its democratic institutions. The country has maintained a stable democratic system since then.
Liberia was founded by freed American slaves and adopted a constitution inspired by the U.S. Constitution; however, it has faced significant challenges and instability over the years.
Spain transitioned to democracy after the Franco dictatorship but has had a shorter history of stable democratic governance compared to Germany.
France has experienced multiple republics and changes in government structures, leading to periods of instability.
Thus, the best answer indicating long-term success with adopting democracy would be Germany.