Which of the following best describes public goods?

Public goods are best described as non-excludable and non-rivalrous. This means that once they are made available, no one can be effectively excluded from using them, and one person's use does not diminish the ability of others to use them as well. Classic examples of public goods include clean air, national defense, and public parks.

Non-excludability means that it is difficult or impossible for a provider of the good to prevent individuals from accessing it, even if they do not pay for it. Non-rivalry implies that the consumption of the good by one individual does not reduce its availability for consumption by others. These characteristics often lead to challenges in market provision since private entities may be discouraged from producing them due to the inability to charge consumers directly.

In contrast, the other options describe different characteristics that do not apply to public goods. For instance, exclusive and rivalrous goods are typical of private goods, while the idea of highly regulated goods might pertain more to common resources or specific market failures rather than the fundamental nature of public goods. Lastly, the availability of a good to certain citizens implies exclusivity, which contradicts the definition of public goods.