Final answer:
An immigrant working hard to leave poverty and becoming middle class is an example of social mobility.
Step-by-step explanation:
Social mobility refers to the ability of individuals to change positions within a social stratification system. Based on Kennedy's definition, an example of social mobility is option d: An immigrant working hard to leave poverty and becoming middle class. This is an example of upward mobility, as the immigrant is improving their economic status and moving to a higher social class.