Which of the following best defines public goods?

Public goods are best defined as goods that are non-excludable and non-rivalrous. This means that once these goods are made available, it is not possible to prevent anyone from using them, and one person's use of the good does not reduce its availability to others. Classic examples include clean air, national defense, and public parks. Because of these characteristics, public goods can lead to issues like free-riding, where individuals benefit from the good without contributing to its provision.

The other options describe characteristics that do not align with the concept of public goods. For instance, goods available only to specific groups imply exclusion, contradicting the non-excludable nature of public goods. Limited availability suggests rivalry, which means one's consumption diminishes another's ability to consume, again countering the non-rivalrous aspect. Lastly, stating that goods can be purchased at any price does not pertain to the defining traits of public goods, as they are not typical market products bought or sold at variable prices.