Which event caused a decrease in trans-saharan trade
a. the settlement of timbuktu
b. the ban on slavery in africa
c. the collapse of the umayyad caliphate
d. the increase in coastal trade with portugal

The correct answer is letter D. the increase in coastal trade with Portugal. It caused a decrease in Trans-Saharan Trade. Trans-Saharan trade needs to travel across the Sahara until you will reach the sub-Saharan part of Africa. Its peak of trade runs from the 8th century towards the early 17th century. The Portuguese journeyed towards the coastal areas of West Africa, opening new avenue in trade..

Yes that is correct answer in 6