Final answer:
The public health nurse is demonstrating advocacy by lobbying for increased funding for health screenings for migrant farm workers.
Step-by-step explanation:
The concept that the public health nurse is demonstrating by lobbying Congress to increase funding for health screenings for migrant farm workers is advocacy. Advocacy involves actions taken to influence public policy in a way that improves conditions or positively impacts the welfare of the public. In the context of health care, advocacy can focus on securing funding for research, education and prevention programs.