Final answer:
Humanistic therapy is most effective for treating people with relationship problems as it is designed for issues centered on personal growth and self-actualization, rather than medical conditions like schizophrenia or bipolar disorder which often require medication.
Step-by-step explanation:
Humanistic therapy focuses on individual personal growth and self-actualization and is considered to be most effective for treating issues rooted in personal perceptions and internal conflicts, such as people with relationship problems. It is less suited for medical conditions like schizophrenia or bipolar disorder, which often require medication and more structured treatments due to their biological and neurological underpinnings. Humanistic therapy might be beneficial in the context of a comprehensive treatment plan for these conditions, particularly when addressing the patient's emotional well-being, but it is not the primary treatment modality for them. Therefore, the correct answer to the question is 'c. people with relationship problems.