The correct answer is D) It served to unify the empire.
Islamic law affected the Islamic Empire in that it served to unify the empire.
Islamic Law is an important component of the Muslim's religion. It includes the concept of "Sharia" that means "the correct path," that comes from the Islam sacred book called the Koran or Quran. So through the teachings of Prophet Muhammad, Muslims learn about religious principles. This Islamic law served to unify the Empire and became the rule for behavior and the path to Allah.
The other options of the question were a) it caused problems with communication, b) it created hostility and conflicts, and c) it negatively affected Islam's trade relations.