The correct answer is: D. Eiffel proved that the Eiffel Tower could benefit scientific research.
While all the options have some relevance, the Eiffel Tower was initially built as a temporary structure for the 1889 Exposition Universelle (World's Fair). However, it was allowed to remain because Gustave Eiffel and his team demonstrated its value for scientific research, including meteorological studies and radio transmission. This scientific utility helped justify its permanence, eventually leading to its iconic status.