Which of the following best defines feudalism?

The best definition of feudalism is a system based on land for service. This social and economic structure was prominent in medieval Europe, particularly after the decline of the Roman Empire. In feudalism, land (known as a fief) was granted by a lord to vassals in exchange for military service and other obligations. This relationship created a hierarchical system where loyalty and protection were exchanged for land and labor.

Lords would provide land to vassals, who in turn would pledge their loyalty and military support. This system also extended to peasants, who worked the land and provided agricultural produce in exchange for protection and the right to work on the land. The relationships defined by feudalism were crucial to the organization of medieval society and the economy, providing stability during a time marked by invasions and political fragmentation.

The other options, while they describe other concepts, do not encapsulate feudalism's unique characteristics. An economic system based on trade refers more to mercantilism or trade economies, a type of monarchy involves different aspects of governance that do not necessarily correlate to the feudal obligations between lords and vassals, and a religious doctrine pertains to beliefs and practices in faith systems rather than social and economic relations.