The correct answer to this open question is the following.
The statement that best describes the spread of culture along east Africa's Swahili coast is "African culture mingled with traditions carried by Arab traders."
Africa's Swahili were known to be good traders. In those years, the Swahili were a tribe that inhabited the territories of modern-day Mozambique, Kenya, and the south part of Somalia. Throughout the Swahili coast, they developed trade routes to export their raw materials successfully. Indeed, trade activity was so important for the Swahili in those years that the routes they developed were known as the Indian Ocean Trade.