A public good is non-excludable, meaning that it is difficult or impossible to prevent people from consuming it, even if they do not pay for it.

The underprovision of public goods is primarily due to the presence of free riders, who benefit from the good without contributing to its provision.

National defense is a public good because it is non-excludable and non-rivalrous, meaning that everyone benefits from it and no one can be excluded from its consumption.

Free riding is the term used to describe the situation where individuals benefit from a public good without contributing to its provision.

All of the above options can be potential solutions to the problem of free riding in the provision of public goods.

A negative externality is a situation where the consumption of a good or service by one individual imposes a cost on others.

Pollution is an example of a negative externality because it imposes a cost on others, such as health problems or environmental damage.

A positive externality is a situation where the consumption of a good or service by one individual generates a benefit for others.

Education is an example of a positive externality because it generates benefits for society as a whole, such as increased productivity and social mobility.

The tragedy of the commons is a situation where a common resource is overused because individuals have an incentive to use it excessively.

Overfishing is an example of the tragedy of the commons because each individual fisherman has an incentive to catch as many fish as possible, even if it means depleting the fish population.

Collective action is the situation where individuals cooperate to provide a public good or service, even though they could benefit from free riding.

All of the above options are examples of collective action, where individuals cooperate to provide a public good or service.

All of the above options are challenges associated with collective action.

All of the above options can be used to promote collective action.