Final answer:
The Byzantine and Persian empires were at war during much of Muhammad's prophetic career. Their conflict weakened both empires, paving the way for the Arab-Muslim conquests and the establishment of the Islamic caliphate. So, the correct answer is option a.
Step-by-step explanation:
The two empires that were at war during much of Muhammad's prophetic career were the Byzantine and Persian empires. This extended conflict was between the Byzantine Empire, also known as the Eastern Roman Empire, and the Sasanian Empire of Persia. The wars between these two great powers flared up at the beginning of the 7th century, leaving both empires exhausted, which in turn contributed to the rapid expansion of the Arab-Muslim conquests shortly thereafter.
Detailed records show that the Byzantines and Sasanians were involved in regular skirmishes and political conflicts, with their relationship being particularly tense due to ambitions for territorial expansion. This tumultuous period provided an opportunity for the Arabs, united under the leadership of Muhammad, to extend their influence and create the new Islamic caliphate, an empire spanning from the Iberian Peninsula to the Indus River valley.
During the reign of the second caliph, Umar, significant military campaigns expanded the caliphate's territory into former Byzantine and Persian lands, after which Umar implemented administrative reforms to manage this vast empire effectively.
So, the correct answer is option a.