The option that best illustrates the impact of trade in East and Southern Africa between the 800 and 1400s is:
A: A people who live along the coast of East Africa spoke Swahili.
This is because the Swahili language itself developed as a result of trade interactions between the coastal Bantu-speaking populations and Arab traders, which facilitated cultural exchange and commerce along the East African coast during that period. The other options do not specifically relate to the significant impacts of trade during that timeframe.