The feudal system developed primarily as a response to instability and insecurity in medieval Europe, particularly due to external threats such as Viking invasions. As kings struggled to maintain control and protect their territories, local nobles offered security and protection to the people in exchange for loyalty and military service. This arrangement involved granting land (or fiefs) to vassals who would then serve the nobles as knights.
While the other responses touch on important aspects of societal and political changes in Europe, the core reason for the emergence of feudalism revolves around the need for local protection and governance during a time of chaos and fragmentation. Thus, the first response accurately captures the fundamental reason for the development of the feudal system.