Question

What trade good most stimulated the trans-Saharan trade between Songhai and North Africa?

tea

gold

spices

copper

Answer 1

this was in the Wikipedia article The Darb el-Arbain trade route, passing through Kharga in the south and Asyut in the north, was used from as early as the Old Kingdom for the transport and trade of gold, ivory, spices, wheat, animals and plants for the trans Saharan trade so i would go with either gold or spices but this link looks like it may have your answer if you want to read through it http://www.pearsonhighered.com/assets/hip/us/hip_us_pearsonhighered/samplechapter/0205835473.pdf
spices and gold are both good options and both are found in Africa
Hope this helps you find the right answer :)