Final answer:
The underlying assumption of 'What happens in the group stays in the group' is Confidentiality.
Step-by-step explanation:
The underlying assumption of 'What happens in the group stays in the group' is Confidentiality (option B).
In group therapy, it is important for participants to feel safe and trust that the information shared within the group will remain confidential. This encourages honesty and open communication.
Cognitive Behavioral Therapy (option C) focuses on changing negative thought patterns and behaviors, and a code of ethics (option D) outlines professional standards and guidelines for various professions.
Therefore, the best choice that aligns with the given assumption is Confidentiality (option B).