The residual powers retained by each state to safeguard the health and welfare of its citizens is the concept to address in regard to the reference to police power.
Option a
Explanation:
The state's main purpose is to safeguard and protect its citizens from any harm that may be coming in their way. It is there for the welfare of its citizens and for their own good. The state protects its citizens from harms that can come in many ways. It can also impose civil laws and public policies, or police support to cope up with any difficulties.
In the given example, Mary Dogood imposes a ban on junk foods within 500 feet from the school, for a good cause that would save the children from indulging themselves in unhygienic and unhealthy foods, that may harm them and make them fall ill. The police power exercised its role to impose this law correctfully to protect the citizens.