Why does the code mention both civil law and law of nations

Civil law, civilian law, or Roman law is a legal system originating in Europe, intellectualized within the framework of late Roman law, and whose most prevalent feature is that its core principles are codified into a referable system which serves as the primary source of law. This can be contrasted with common law systems whose intellectual framework comes from judge-made decisional law which gives precedential authority to prior court decisions on the principle that it is unfair to treat similar facts differently on different occasions (doctrine of judicial precedent, or stare decisis).[1][2]

Historically, a civil law is the group of legal ideas and systems ultimately derived from the Codex Justinianus, but heavily overlaid by Napoleonic, Germanic, canonical, feudal, and local practices,[3] as well as doctrinal strains such as natural law, codification, and legal positivism.

Conceptually, civil law proceeds from abstractions, formulates general principles, and distinguishes substantive rules from procedural rules.[4] It holds case law to be secondary and subordinate to statutory law. When discussing civil law, one should keep in mind the conceptual difference between a statute and a codal article. The marked feature of civilian systems is that they use codes with brief text that tend to avoid factually specific scenarios.[5] Code articles deal in generalities and thus stand at odds with statutory schemes which are often very long and very detailed.

the answer is option a.