What event signaled the end of Swahili coastal dominance over maritime trade?
A) The rise of Mali on the West coast contributed to the fall of the Swahili coast empires.
B) The arrival of the Portuguese signaled the end of maritime dominance along the Swahili coast.
C) A lack of trading partners across the Indian Ocean contributed to a decline in wealth and power along the Swahili coast.
D) Disease spread through the coast brought from Indian trading partners. As a result, the coastal kingdoms began to decline.

This is the prime reason.

B) The arrival of the Portuguese signaled the end of maritime dominance along the Swahili coast.

Explanation:

The Swahili had dominated the coast and maritime trade of Africa for ages and had been a developed society there.

This was until the arrival of the Portuguese who wanted to take over the coast from them as only then would they be able to reap true profits of their trade with Asia

Thus, they were able to overthrow the empire and make it their own protectorate and established their own trade from there.

might be agreeing to participate ethically with other european nations in the slave trade

or notifying other european nations when taking possession   of african territories