Concept:
The modern concept of Rule of Law emphasizes that law is supreme and that no individual, including rulers, is above the law. Roman jurisprudence contained several concepts that influenced later legal systems.
Step 1: Understanding Lex Regia.
The expression Lex Regia refers to the authority and legal framework governing the ruler and the exercise of sovereign power.
Step 2: Connection with Rule of Law.
The idea underlying Rule of Law is that governance must derive legitimacy from law rather than arbitrary power. Roman legal thought expressed similar ideas through the concept of Lex Regia.
Step 3: Conclusion.
Among the given options, Lex Regia is considered the Roman concept most closely associated with the modern idea of Rule of Law.
\[
\boxed{\text{Lex Regia}}
\]