The correct option is B Law merchant
Lex mercatoria, often referred to as "the Law Merchant" in English, is the body of commercial law used by merchants throughout Europe during the medieval period. It evolved similar to English common law as a system of custom and best practice, which was enforced through a system of merchant courts along the main trade routes. It functioned as the international law of commerce. It is the most important source of Mercantile Law.