Final answer:
The incorrect characteristic of a public good is that it has no opportunity cost because it is free. Public goods are instead defined by being nonexcludable and non-rivalrous, meaning they are available to everyone without one person's consumption reducing availability to others.
Step-by-step explanation:
The characteristic of a public good that is not accurate is a. Because it is a free good, there is no opportunity cost. This statement is not true; even for public goods, resources used are not available for alternative uses, which represents the opportunity cost. Public goods such as national defense or public parks are typically paid for through taxation and cannot be provided without cost.
Public goods have two main defining characteristics: they are nonexcludable and non-rivalrous. Being nonexcludable means it is difficult or impossible to prevent anyone from using the good, like in the case of national defense. Non-rivalrous or non-rival means one person's use of the good doesn't reduce the amount available for others, for example, an additional person listening to a public concert doesn't prevent others from enjoying it too.
However, it is important to recognize that markets can face difficulties in producing public goods due to the problem of free riders -- people who use the public good without contributing financially. Solutions to this challenge include government intervention, social pressures, or unique market approaches to ensure that users help pay for the public goods they benefit from.