The capital city originally known as Léopoldville until 1966 was Kinshasa.
Kinshasa is the capital of the Democratic Republic of the Congo, on the southern bank of the Congo River. During the colonial period, the city was named Léopoldville after King Leopold II of Belgium, whose Congo Free State controlled the territory before Belgian annexation in 1908.
Léopoldville became the capital of the Belgian Congo in 1926, replacing Boma. It remained the capital when the country became independent in 1960. The name was changed to Kinshasa in 1966 as part of a broader policy of Africanization and the removal of colonial-era names. The new name referred to an older local settlement associated with the site.
The alternatives are especially plausible because the region has two prominent Congolese capitals. Brazzaville, across the Congo River, is the capital of the Republic of the Congo and was historically distinguished as Congo-Brazzaville. Kisangani and Lubumbashi are major cities in the Democratic Republic of the Congo, but neither was known as Léopoldville.