Final answer:
Rubber was one chief export of the Free State of Congo.
Step-by-step explanation:
One chief export of the Free State of Congo was rubber. Europeans in the Congo Free State exploited the local population to harvest rubber from wild rubber tree vines. The laborers would chop through the vines and let the liquid latex cover their bodies, which hardened into rubber that had to be scraped off at the end of the day.
Learn more about Exports of the Free State of Congo