Final answer:
Justinian made significant improvements to Constantinople by constructing the Hagia Sophia, introducing a new legal code, and enhancing the city's fortifications. The correct option is D.
Step-by-step explanation:
After the Nika riots were put down, Justinian made several notable improvements to Constantinople. First, he ordered the construction of the Hagia Sophia, an architectural masterpiece that served as a central church in Constantinople.
Additionally, Justinian's legal code, often known as the Corpus Juris Civilis or the Code of Justinian, was introduced to unify and simplify the complex and overlapping laws of the empire. Furthermore, improvements were made to the city's fortifications, including its walls, to protect the city from future attacks and invasions.
These improvements were monumental in enhancing the grandeur, security, and legal sophistication of Constantinople during Justinian's reign.