Final answer:
The major conflict between the Ottoman and Safavid Empires was rooted in 'Religious differences,' with the Ottomans being Sunni and the Safavids being Shia Muslims, leading to deep-seated sectarian tensions.
Step-by-step explanation:
The major source of conflict between the Ottoman and Safavid Empires was a) "Religious differences". The Ottomans were a Sunni Muslim empire, while the Safavids were Shia Muslim. This sectarian split was a significant source of strife between the two powers, as each empire had differing views on Islamic leadership and the legitimacy of authority within the Muslim world. The rivalry was exacerbated by territorial disputes as both empires sought to expand their territory, but the fundamental issue stemmed from their religious discord, with the Safavids enforcing Shia Islam as the state religion and the Ottoman using the millet system to govern their diverse population.