During the rule of Emperor Justinian, the Corpus juris civilis was important because it

The Corpus Juris (or Iuris) Civilis ("Body of Civil Law") is the modern name for a collection of fundamental works in jurisprudence, issued from 529 to 534 by order of Justinian I, Eastern Roman Emperor ... The work was directed by Tribonian, an official in Justinian's court.

C) allowed Byzantine citizens to know what law governed them.

Explanation:

The Corpus juris civilis (Body of Civil Law) was important to the Byzantine world because it was a codification - listing and detailing - of all of the laws of the empire. Therefore, it allowed Byzantine citizens to know what law governed them. This was a major issue, since many laws had changed over the centuries and had not been properly written down.