The best response that compares 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.
This response accurately reflects the historical context: the Sasanian Empire was significantly weakened by internal strife and external pressures, leading to its rapid decline after the rise of Islam. Meanwhile, although the Byzantine Empire lost significant territories, it was able to maintain some degree of stability and resilience over the centuries that followed, despite facing ongoing challenges from Islamic forces.