The bubonic plague had a significant impact on labor and the role of peasants in feudal society. Before the plague, the feudal system was characterized by a rigid social hierarchy where peasants worked the land in exchange for protection and a small portion of the produce.
After the plague, which caused a drastic reduction in the population, the balance of power shifted. The labor shortage that resulted allowed surviving peasants to demand higher wages and better working conditions. This increase in demand for labor enabled many peasants to gain greater social mobility, as they could negotiate better terms for their work or even move to different areas in search of better opportunities.
Thus, the most relevant impact in the context of your options is increased social mobility.