The correct answer would be option A, Confidentiality of the individual subject's responses.
Explanation:
When a researcher invites a therapist to participate in a small focus group to discuss the perception about the troubled adolescent girls and the relationships they have with their parents. People usually don't discuss their such private problems in public and don't want to disclose their identity when discussing such problems on forums like focus groups where some therapist is present to listen to their problems. So confidentiality is a crucial concern in this. Instead of watching for the breach of confidentiality from the focus group participants, the researcher must focus on maintaining the confidentiality of the group members.