Final answer:
The clinic provided by a PHN for HIV-positive individuals represents tertiary prevention, focusing on managing the disease and improving patients' quality of life.
Step-by-step explanation:
The activity of a Public Health Nurse (PHN) providing a clinic for HIV-positive citizens in the community is best described as tertiary prevention. Tertiary prevention aims to manage and improve quality of life for individuals with a disease by preventing complications, managing pain, providing treatment, and limiting disability.
This differs from primary prevention, which seeks to prevent the onset of disease, and from health education, which could be categorized as a part of primary or secondary prevention depending on context. While policy making can influence all stages of prevention, the PHN's direct care through a clinic is a form of intervention rather than creating policy.