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

National defense is an example of a public good because it possesses two key characteristics: non-excludability and non-rivalry. Non-excludability means that individuals cannot be effectively excluded from using the good; once national defense is provided, it protects everyone within the nation regardless of whether they contribute to its funding or not. Non-rivalry indicates that one person's use of national defense does not diminish its availability to others; the protection provided is equally available to all citizens without reducing the level of security for anyone.

In contrast, private parks, rivalrous food items, and national toll roads do not share these characteristics. Private parks typically require payment or membership for access, making them excludable, and they can become overcrowded, reflecting rivalry among users. Rivalrous food items are limited in nature—when one person consumes food, it reduces the amount available for others. National toll roads are also excludable because drivers must pay to use them, and their capacity can be limited, creating competition among users. These aspects differentiate public goods like national defense from other examples.