The Eiffel Tower was classified as a historical monument in 1964.
France officially designated the tower a monument historique in 1964, during the tenure of André Malraux as minister of cultural affairs. The protection recognized that the structure was no longer merely a temporary exhibition attraction but a major part of France’s architectural and national heritage.
The tower had originally been built for the 1889 Exposition Universelle, marking the centenary of the French Revolution. Construction ran from 1887 to 1889, and the iron lattice design initially provoked strong criticism from artists and intellectuals. Its monument status came well after the exhibition, and should not be confused with its later inclusion in the UNESCO World Heritage Site “Paris, Banks of the Seine” in 1991.
The tempting dates in this question refer to different milestones: 1889 was the opening year, 1900 was another Paris world’s fair, and 1957 saw the installation of the tower’s modern radio antenna.