Final answer:
The Assumption of growth is the Lifespan perspective concept that relates to the capacity of people and families to change over time, aligned with the stages of the family life cycle.
Step-by-step explanation:
From a Lifespan perspective, the assumption that relates to the capacity of people (and families) to change throughout time is the Assumption of growth. This concept posits that individuals and families undergo a process of growth and development, which encompasses a wide range of physical, cognitive, social, and emotional changes. The family life cycle is a framework that has been used to describe these changes. This cycle includes stages such as marriage, parenthood, the 'empty nest' period, and old age, each presenting unique challenges and opportunities for growth.