Final answer:
The most lasting accomplishment of Emperor Justinian I was the Corpus Juris Civilis, a codification of Roman law that influenced many European legal systems. The correct option is option (a).
Step-by-step explanation:
Emperor Justinian I of the Byzantine Empire is remembered for his significant contributions in various areas, but his most lasting accomplishment was the Corpus Juris Civilis, also known as the Code of Justinian. This monumental work codified Roman law, creating a legal system that served as the basis for many European legal codes. Justinian's other notable achievements included his military campaigns, such as his victory over the Ostrogoths in Italy and efforts against the Persians, as well as his role in Christianizing the empire and enforcing religious uniformity. Yet, it is the Code of Justinian that has had the profoundest and most enduring influence on the world, establishing the foundation for modern law in numerous countries.
While Justinian's reconquest of the western parts of the former Roman Empire and his military victories were significant, they were temporary and ultimately unsustainable. His construction projects, like the Hagia Sophia, also left a lasting architectural legacy. However, his legal reforms and the establishment of the Justinian Code had a more direct impact on the development of legal systems and the course of European history. The choice in the question, the construction of Notre Dame, can be discounted as it was not constructed during Justinian's reign, and he also did not directly defeat the Muslim armies outside Constantinople as they did not rise to prominence until after his time.