Final answer:
The correct term to complete the sentence is 'developmental,' relating to the multiple developmental trajectories in the context of family life. The family life course is a flexible sociological model that better reflects the diverse nature of modern family life.
Step-by-step explanation:
The concept of multiple developmental trajectories involves the timing of various life events. This idea is often explored within the framework of the family life course, a sociological model that views family progression as a fluid series of events rather than adhering to strict stages. This model contrasts earlier theories, which tended to generalize family life and ignored variations based on gender, ethnicity, culture, and lifestyle. The family life course model is more in tune with today's society, where, for example, childbearing does not always occur within marriage, reflecting changes in family development and shifting practices of family life.