Which of the following is an example of a public good?

National defense serves as a clear example of a public good due to its defining characteristics: it is non-excludable and non-rivalrous. This means that once national defense is provided, it protects all citizens without the possibility of excluding anyone from its benefits, and one person's benefit from national defense does not diminish another person's enjoyment of that same security.

In contrast, luxury hotels, private housing developments, and specialized health services are all examples of private goods. These goods are excludable and rivalrous; individuals can be prevented from using these services if they do not pay for them, and usage by one individual can reduce availability for others. This distinction underscores the unique nature of public goods like national defense, which requires collective funding and participation beyond the individual level.