Answer:
A) Justinian Code
Explanation:
The Civil Law, Common Law, or Islamic Law are the three main sources of law in the world today.
Roman Law served as the foundation for the Civil Law. Law was viewed as personal rather than territorial in the defunct Western Empire. Thus, during the rule of the germanic monarchs, "vulgar roman law" continued in Italy where Italians were still ostensibly subject to the same rules. According to Gratian's work, Canon Law had a significant role in the formation of the Civil Law, which also drew on Roman Law for its principles and practices.
But the Digest—the bulk of Justinian's Code—was crucial to the Civil Law. The Digest was found in Italy and extensively studied by the University of Bologna's Glossators, who later inspired other generations of academics throughout Europe and Italy. Though they are frequently criticized for having created a relatively ahistorical vision of the law, the academics endeavored to resolve contradictions, to defend existing practices, and to recreate the language and intellectual background of the Digest.
The Digest-derived law was adopted into national law from European universities. It was formally promulgated in the HRE. It was supposed that it was still in force elsewhere. Others embraced it without any kind of express enactment and viewed it as "learned law." More information about this subject may be found in just about any book on the development of law in Europe (or a specific European nation).
The scholastics and later humanist and natural law philosophers also embraced the Digest-related studies. The authors of Latin treatises like Pufendorf and Grotius—two titans of international law—clearly carry on the Digest's legacy. The Latin authors are followed by treatise authors in various national languages. It takes just a short detour from the final French writers in the natural law tradition to the French Civil Code, which enacts their works.
The rest of Europe also adopts laws in the century that follows the adoption of the French Civil Code. A few are taken straight from the French Code. Some are a blend, including aspects of regional law and local romanistic heritage.
Later, Japan adopted the Civil Law, which also served as the foundation for the Communist legal systems.
Less directly, the Digest has an impact on common law. The Inns of Court in England, who were supporters of the traditional legal system, which had developed in a way that (at least procedurally) more closely resembled the classical Roman Law than the more imperial version that Tribonian would have known, controlled a large portion of the country's legal system. Although the "learned law" was not acknowledged in English courts, English colleges would have provided teaching in it.
But Justinian's Code did have an impact. Bracton, Coke, Hale, and other English treatise authors have obviously studied the code or read authors who have. Similar sections in Justinian serve as the inspiration for important passages in these English writers that discuss the sources and goals of law. Even if the substance of these works are occasionally very diverse, the arrangement of them does appear to be derived from the code.
It is also clear that English law would borrow concepts from the European jus commune (the nascent Civil Law) sometimes awkwardly. For instance, English law lacks the all-important distinction between ownership and possession found in Roman law -- possession in English law is a practical concept (you have it) not a technical one. Thus, when English law borrows Roman property law (which was quite sophisticated) it must sometimes create strange kludges. English law also lacked the elegance of Roman law with respect to easements, and has really only settled at the same point as the Digest in the past few decades.
My knowledge skips a few centuries at this point, but by the time we get to America, common lawyers are citing "the Civil Law" frequently, and what they mean by this is a very influential translation of Justinian's Institutes (not the Digest). The Institutes were particularly seen as relevant in the context of the American law of slavery, because English law lacked a suitable antecedent. But even in more mundane cases, Justinian is cited, and during the 19th century it would be easy to purchase a copy of the Institutes with footnotes to American decisions, just as one could purchase many abridgments of Blackstone with footnotes to American decisions.