This is the prime reason.
B) The arrival of the Portuguese signaled the end of maritime dominance along the Swahili coast.
Step-by-step 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.