Which step did Britain take to prevent rebellion in its colonies during the nineteenth century?

If you mean the 1900's, you may be looking for the creation of Commonwealth realms - Britain granted Canada, Australia, and some other places legislative independence but kept them under Britain's constitutional monarchy. So they were essentially free, but they would be encouraged to remain an ally of Britain.

To avoid bloodshed, 12 European powers met at an international conference in 1884 in Berlin, not Africa. US & the Ottoman Empire were invited, but no Africans. Set the ground rules for colonization. They established new borders & frontiers. They redrew the map of Africa with little regard for traditional patterns of settlement or ethnic boundaries.