Floodplains are either formed by erosion or by aggradation.
People might want to live here, because the sediments in the river make the soil rich, which is good for farming. There is also plenty of water around because of the rivers, and the rivers can be a form of transportation. The river can also be a food source.
The Columbian Exchange greatly affected almost every society on earth, bringing destructive diseases that depopulated many cultures, and also circulating a wide variety of new crops and livestock that, in the long term, increased rather than diminished the world human population
B. sedimentary ...hope this helps