Final answer:
Public Health Nurses implementing registries to track and follow up on children's vaccinations are engaged in d. individual level practice.
Step-by-step explanation:
Public Health Nurses (PHNs) use registries to identify children with delayed or missing immunizations and perform follow-ups with families through phone calls or home visits. This represents an implementation of the d. individual level of practice in the public health system.
At this level, health professionals engage directly with individuals or families to address specific health needs or concerns, such as updating vaccinations.