The fundamental division in the Islamic world was The division between Sunni Muslims and Shiite Muslims The division between the military and the civilian population The division between the aristocracy and the common people The division between Christians and Muslims

The division between Sunni Muslims and Shiite Muslims

Explanation:

The fundamental division in the Islamic world was "the division between Sunni Muslims and Shiite Muslims."

This is evident in the fact that the division between the two groups started as far back as the 14th century in which there was a struggle over who to succeed the Prophet Muhammad.

According to some credible statistics, the 85% of Mislims are Sunnis while the the remaining 15% are Shiite.