Final answer:
Iberville decided to erect a fort at English Turn to protect French holdings along the Mississippi after recognizing its strategic importance in controlling trade and colonization efforts in the area.
Step-by-step explanation:
The incident that convinced Iberville that a fort should be erected to protect the French holdings along the Mississippi River was at English Turn.
This location was strategically significant because it served as a turning point on the river near New Orleans where a hostile force, such as the British fleet, could potentially take control of the Mississippi and disrupt French colonization and trade.
The construction of a fort was deemed necessary to safeguard French interests and maintain their presence in the region.