What is an example of a public good in economic theory?

In economic theory, public goods are characterized by being non-excludable and non-rivalrous. This means that individuals cannot be effectively excluded from using them, and one person's use does not diminish the availability for others. National defense is a prime example of a public good because it provides security to all citizens within a nation regardless of their individual contributions or participation. Since national defense protects everyone, it is impossible to exclude anyone from its benefits, and one person's safety being ensured does not take away from another's.

In contrast, the other options involve characteristics that are not aligned with public goods. Private parks are excludable because access can be restricted to certain individuals. Cellular data services and personal tutoring services are both excludable and rivalrous—if one person is using a cellular data service, it can limit the service available to others, and personal tutoring services are limited to those who can afford to pay for them. Thus, national defense stands out clearly as an exemplar of a public good in economic theory.