Final answer:
The Magna Carta and the Model Parliament led to the decline of feudalism by establishing legal principles that limited monarchical power and by promoting centralized government, thus reducing the influence of feudal lords and enabling social mobility for commoners.
Step-by-step explanation:
Political events like the Magna Carta and the development of the Model Parliament played significant roles in the decline of feudalism. The Magna Carta, signed in 1215, established the principle that everyone, including the king, was subject to the law, laying a foundation for due process and the limitation of monarchical power. The Model Parliament, first convened in 1295, brought together nobles, clergy, and commoners, expanding representation and paving the way for a more centralized government.
Feudalism weakened as this increasing centralization of power reduced the influence of the feudal lords. Additionally, socio-economic factors, such as the Black Death and the Hundred Years' War, shifted military practices and diminished the roles of traditional feudal elites. The rise of professional armies offered peasants the potential for social mobility and bolstered centralized monarchical power, further eroding the feudal system.
The question's answer concerning the Magna Carta's principle is d. that all people, even the king, were subject to the law, which signified a transition from arbitrary rule to one of greater accountability and the rule of law.