John worked as a software engineer for a large technology company for over three decades. Upon reaching the age of $65,$ he retired from his full$-$time position but continued to stay involved in the field of technology. Instead of completely stepping away from his career, John decided to start his own software consulting business from home. His adjustment to retired life can be seen as an example of the $\_\_\_\_\_\_\_\_\_\_$ theory.

$$a$.$

normative

b$.$

continuity

c$.$

selection

d$.$

chronological