Final answer:
The use of camels as transportation was essential for the expansion of the gold and salt trade in West Africa.
Step-by-step explanation:
The technological invention that was essential to stimulate the expansion of the gold and salt trade in West Africa was the use of camels as transportation in the trans-Saharan trade routes.
Camels were able to withstand the harsh desert conditions and carry heavy loads of goods, such as salt, across the Sahara Desert. This allowed for efficient trade between communities on the edges of the desert and those in the forest regions to the south that had access to goldfields.
The use of camels facilitated the growth of the gold and salt trade, enabling the exchange of valuable commodities and the development of West African kingdoms.