Final answer:
The first Fatimid Caliph was neither Imam Ali, Imam Jafar al-Sadiq, Imam Hassan, nor Imam Ismail. It was in fact Abu Muhammad Abdullah, known as Imam al-Mahdi.
Step-by-step explanation:
The first Fatimid Caliph was Abu Muhammad Abdullah, who established the Fatimid Caliphate. This caliphate was the only Shia caliphate and claimed descent through Ali's wife and the Islamic prophet Muhammad's daughter, Fatima. The Fatimids claimed spiritual authority as imams based on their biological descent from Ali, reinforcing the principle that Shia leadership should come from Muhammad's direct line. Thus, the answer to the question of which Imam was the first Fatimid Caliph is not directly listed among the options provided to the student (Imam Ali, Imam Jafar al-Sadiq, Imam Hassan, Imam Ismail). The correct answer is none of the options given; it is Abu Muhammad Abdullah who was known as Imam al-Mahdi.