What are some examples of a public good?

Public goods are defined by two main characteristics: they are non-excludable and non-rivalrous. This means that once a public good is made available, it is difficult or impossible to prevent individuals from using it, and one person's use does not diminish the availability of the good for others.

The specified examples of clean air, national defense, and public parks fit these criteria perfectly. Clean air is available for everyone to breathe, and one person’s use does not prevent others from using it. National defense is a service provided to all citizens, ensuring protection and security without excluding anyone from its benefits, regardless of their contribution to funding it. Public parks are accessible to anyone without restriction, allowing free enjoyment for all without limiting access based on usage.

In contrast, the other answer choices include items that are generally excludable or rivalrous — such as private schools and toll roads, which require payment and provide services that only the paying individual can access. Similarly, luxury hotels and personal fitness programs cater to specific individuals who pay for the use, demonstrating the qualities of private goods rather than public ones.