Which of the following is an example of a Public Good?

A public good is characterized by two main features: it is non-excludable and non-rivalrous. This means that once a public good is provided, it is available for everyone to use without the possibility of excluding anyone from its benefits, and one person's use does not reduce its availability to others.

National defense exemplifies a public good because once it is established and funded, all citizens benefit from the security and protection it provides, regardless of whether they contribute directly to its funding or not. No individual can be excluded from enjoying the protection offered by national defense, and one person's security does not diminish the security available to others.

In contrast, the other options illustrate characteristics of private goods, which are both excludable and rivalrous. Exclusive concert access, pizza at a party, and restricted access to a swimming pool can all be limited to specific individuals, and one person consuming them does prevent another from doing so. Therefore, these examples do not fulfill the criteria needed to be classified as public goods.