The most accurate response to compare the reactions of the Byzantine and Sasanian Empires to Islamic conquest is:
The Sasanians crumbled due to instability, the Byzantines held out despite losing territory.
The Sasanian Empire was significantly weakened by internal strife and was unable to effectively respond to the Islamic conquests, leading to its rapid downfall. In contrast, the Byzantine Empire managed to maintain a degree of continuity and governance even after losing substantial territory, demonstrating resilience in the face of external pressures.