Final answer:
The concepts mentioned are describing English Common Law, which alongside the Magna Carta, provide the foundation for due process and the rule of law, even for kings, and influencing modern legal systems like that of the United States. Therefore, the correct option is A. the Magna Carta.
Step-by-step explanation:
The statements "All people should be subject to the same rules" and "A set of principles popular amongst most people in an area," popularized by Henry II of England in the 12th Century, are describing English Common Law. The Magna Carta (Great Charter), signed by King John in 1215, is an important foundation of these legal principles, guaranteeing due process and the concept that even the king is under the law.
These principles were so influential that they have made their way into modern legal systems, such as the United States Constitution, affirming rights and due process in the Fifth and Sixth Amendments. The influence of the Magna Carta extended beyond law, potentially impacting the economic status of England as well, as its GDP per capita was second highest in Europe, after northern Italy, in 1348.