Final answer:
The Sassanian Empire was defeated and assimilated into the early Islamic state, contributing to the rapid expansion of the Islamic Caliphate. Option b is the answer.
Step-by-step explanation:
The state that was defeated and assimilated into the early Islamic state was the Sassanian Empire. After facing off in prolonged conflict with its rival, the Byzantine Empire, the Sassanians were weakened by the time the Islamic conquests began. In a series of decisive battles, the early Muslims defeated the Sassanian forces, thereby consolidating their control over Persian territories. This led to the assimilation of the Sassanian Empire into the newly established Islamic Caliphate. Notably, the conquest of the Sassanian lands marked the quick expansion of the Islamic state into areas beyond the Arabian Peninsula, establishing it as a significant power in the region.