What system allowed nobles to grant land to vassals in exchange for military service?

Feudalism is the correct choice because it was a hierarchical system that defined the relationships between lords (nobles) and vassals during the Middle Ages. Under feudalism, nobles granted land, known as a fief, to vassals in exchange for their pledge of loyalty and military service. This system was foundational for the political and economic organization of medieval society. It emphasized personal bonds and obligations, wherein vassals would protect their lord’s interests in exchange for land and protection themselves.

Manorialism, while relevant to the economic structure of the time, deals more specifically with the organization of rural estates and the relationship between lords and peasants rather than the military service aspect that characterizes feudalism. The social contract concept refers to philosophical ideas about governance and societal agreements, which do not apply to the land and military service exchanges typically seen in feudal relationships. Oligarchy represents a form of governance by a small group of individuals, rather than the specific land-based system of loyalty and service seen in feudalism.