Final answer:
Behaviorism is the theory that believes clients should change to fit the environment. It focuses on how external stimuli and reinforcement shape behavior.
Step-by-step explanation:
The theory that believes clients should change to fit the environment is Behaviorism. Behaviorism focuses on how external stimuli and reinforcement shape behavior and believes that individuals' actions are a response to their environment. Clients are encouraged to modify their behavior based on external factors.