So what is the UN predicting to happen in the next 76 years for the DRC (population 109 million) to surpass the US (population 334 million)? The DRC historically doesn't have the stability that would lead to explosive population growth, and with the climate continuing to warm and the DRC being on the equator it's going to feel the effects of climate change particularly severely.

Nigeria (pop 230 million) seems more likely, though it also is going to have the same problems with climate change the rest of the middle latitudes will have.