Social mobility is best defined as:

Social mobility is fundamentally understood as the movement of individuals and groups between different class positions in society. This concept encompasses both upward mobility, where individuals or groups move to a higher socioeconomic status, and downward mobility, where they move to a lower status. Such movements can be influenced by various factors, including education, employment opportunities, economic conditions, and social networks.

While the ability to change one’s income level is related to social mobility, it is more of an outcome or aspect of the broader concept rather than a standalone definition. Improvement in living conditions over time might be a result of social mobility but does not capture the nuances of mobility across social and economic structures. Similarly, the trend of wealth accumulation pertains to economic factors without directly addressing the movement between class positions, which is central to understanding social mobility. Thus, the correct answer is the one that encapsulates the full scope of how individuals and groups can shift within the social hierarchy.