Final answer:
Songhai's downfall was primarily due to the Moroccan invasion in 1591, which led to its collapse after defeating the empire at the Battle of Tondibi.
Step-by-step explanation:
The factor that contributed to Songhai's downfall as a powerful empire was b) Invasion by European Powers.
Struggles within the empire, such as leadership issues after Askia the Great grew older, certainly weakened the state, but the direct military threat came from outsiders.
The Moroccan invasion in 1591, led by the Saadi dynasty, sought the wealth of the Songhai.
Armed with firearms, the Moroccans defeated the Songhai forces at the Battle of Tondibi, leading to the empire's disintegration and eventual collapse.
While internal strife and leadership issues set the stage for disarray, it was the military defeat and political fragmentation following the invasion that directly brought down the Empire.