Final answer:
The core public health function being implemented in this scenario is Assessment, as a PHN collects data and monitors the health status of the population to inform public health interventions and policies.
Step-by-step explanation:
The core public health function being implemented in this scenario is Assessment. A Public Health Nurse (PHN) who collects data and monitors the health status of the population is conducting an assessment of the population's health. Through data collection and monitoring, the PHN identifies health issues, determines their prevalence, and analyzes trends to inform public health interventions and policies.