Final answer:
The most appropriate action would be to report the finding to the healthcare provider.
Step-by-step explanation:
When a nurse notices that an 8-month-old infant has significant head lag during a routine health assessment, the most appropriate action would be to report the finding to the healthcare provider (option c). Head lag at this age is a developmental delay and may indicate muscle weakness or poor muscle tone. The healthcare provider can further evaluate the infant's development and determine if any intervention or further testing is necessary.