What type of goods combine non-rivalry in consumption with non-excludability?

The correct answer is public goods. Public goods are characterized by both non-rivalry and non-excludability. Non-rivalry means that one individual's consumption of the good does not diminish the ability of others to consume it, such as national defense or a public park. Non-excludability means that it is difficult or impossible to prevent individuals from accessing the good, even if they do not pay for it, like the benefits of a lighthouse or clean air.

This unique combination leads to certain challenges in the market. Because individuals cannot be effectively excluded from using the good, there is often a tendency for under-provision of such goods in a free market. As a result, public goods typically require government intervention or collective action for their provision and maintenance.

In contrast, private goods are characterized by both rivalry and excludability, meaning that one person's consumption affects another's and that access can be restricted. Common resources, while also non-excludable, are rivalrous, meaning their consumption by one person diminishes availability for others. Collective goods, while similar to public goods, are not a widely accepted economic classification and can create confusion, as the defining characteristics of public goods are more widely recognized in economic theory.