Answer:
Option: d. the gold-for-salt trade
Step-by-step explanation:
Ghana began to flourish in West Africa during the 6th to 13th century. Trans-Saharan trade in the West helped Ghana to emerge as a Kingdom because it was the supplier of the gold. Metal like gold became one of the commodities Ghana traded with other merchants. The economy of this region where the empire began to develop flourished in trade (import and export). Ghana imported salt and other materials in exchange for gold.