The correct comparison of the reactions of the Byzantine and Sasanian Empires to Islamic conquest among the options provided is:
The Sasanians crumbled due to instability, the Byzantines held out despite losing territory.
This reflects the historical context where the Sasanian Empire faced significant internal strife and was unable to effectively respond to the Islamic conquests, leading to its rapid decline. In contrast, the Byzantine Empire, while it lost significant territory and faced challenges, was able to maintain its core governance and adapt over time, remaining a significant power for several centuries afterward.