Explore the Congo! is a decision available to Belgium.
The requirements are that Congo is a vassal of Belgium and that Belgium has invented Mission to Civilize.
The decision renames 3 provinces: Bunkeya to Elisabethville, Kinshasa to Leopoldville and Kisangani to Stanleyville. It also gives Belgium 10 prestige.
There are no way to rename the provinces back to the original names, even if Congo gains independence. Despite what would be logical, the decision does not strictly require that neither Belgium nor Congo actually owns the 3 provinces in question, only that both countries exist and that Congo is a vassal of Belgium.
Flavour text
With the Congo Free State having been confirmed as The Personal Property of King Leopold II, it is only fair and reasonable that we now explore the region, and perhaps leave a bit of a personal mark on it?
Even if Belgium changes its government form away from an absolute monarchy, the decision is still available, and the flavour text will not change.