Final answer:
Swahili Coastal cities were known for trading in d. ivory and tortoiseshell, which were used to craft decorative items.
Step-by-step explanation:
Swahili Coastal cities offered a variety of goods, among which d. ivory and tortoiseshell were prominent items of trade. These cities, as part of the Swahili coast, had become important commercial centers by the tenth century, leveraging both overland and maritime trade routes to engage in commerce with regions as far as Arabia, Persia, and India.
Goods such as gold, iron, copper, valuable hardwoods, and animal hides were also traded, but it was specifically ivory and tortoiseshell that were renowned export items used to make decorative objects like combs.