Which of the following best defines a public good?

A public good is characterized by two main properties: non-excludability and non-rivalry. Non-excludability means that it's difficult to prevent anyone from using the good, while non-rivalry indicates that one person's use of the good does not diminish its availability for others.

Option C correctly identifies that a public good is one that cannot be exclusively owned or restricted in use. This aligns with the examples of public goods, such as clean air, national defense, or public parks. Once such goods are provided, they are available for everyone to use, regardless of whether they contribute to their provision.

In contrast, other choices do not accurately capture the essence of a public good. For example, a good that can be easily sold in the market typically has exclusive property rights associated with it, which is contrary to the nature of public goods. Similarly, a good or service with established individual property rights would imply that it is excludable, thus not fitting the definition of a public good. Lastly, a good that is only available during certain seasons does not relate to the characteristics of public goods, as seasonal availability does not inherently address issues of exclusivity or rivalry.