Final answer:
Asking the client about smoking habits would most likely reveal if they are behaving against their treatment plan.
Step-by-step explanation:
The medical history question that would most likely reveal whether a client is behaving in a way that works against their treatment plan is option a) asking the client about smoking habits. Smoking is the major cause of COPD, so if the client is still smoking despite their diagnosis, it indicates that they are not adhering to their treatment plan and are engaging in behavior that worsens their condition. By asking about smoking habits, the nurse can assess the client's willingness to quit and provide appropriate interventions and support.