Final answer:
The counselor should respond with option d, highlighting the seriousness of the situation and the importance of the client's sobriety in their relationship.
Step-by-step explanation:
The best response by the counselor to a client whose wife has vowed to leave him if he resumes drinking would be option d, "she's really serious about your sobriety." This response acknowledges the severity of the issue without judgment and emphasizes the importance of sobriety in the relationship. It also opens the door to a conversation about how the client's drinking affects his marriage, which could lead to a productive discussion about treatment and understanding the underlying issues contributing to the client's alcohol use.