The statement that best describes the differences between Song Dynasty China and Western Europe during the Middle Ages is:
"Song China had a merit-based civil service and greater social mobility, while Europe was emerging from chaos with limited opportunities for advancement."
During the Song Dynasty (960-1279 AD), China developed a civil service examination system that allowed individuals to attain government positions based on merit rather than birth, leading to a relatively higher level of social mobility. In contrast, Western Europe during the early Middle Ages was characterized by feudalism, which often restricted social mobility and was marked by a lack of centralized authority in many regions.