Questions: ¿Cuál de las siguientes afirmaciones es correcta? Select one: a. Un bien público debe ser no excluible. b. Si un bien no es rival, el costo de que lo consuma una persona adicional es cero. c. Algunos bienes públicos son rivales. d. Un bien no puede ser a la vez rival y no excluible.

The answer is a. Un bien público debe ser no excluible.

This statement is correct. A public good is characterized by being non-excludable, meaning that it is not possible to prevent people from using it. Examples include public parks and national defense.

This statement is also correct in the context of non-rival goods. Non-rival goods can be consumed by additional people without increasing the cost. However, this option is not as directly defining a public good as option a.

This statement is incorrect. Public goods are typically non-rival, meaning one person's use does not reduce availability to others. If a good is rival, it means that one person's consumption reduces the amount available for others, which is not a characteristic of public goods.

This statement is incorrect. A good can be rival and non-excludable, such as a common resource like fish in the ocean. These are known as common-pool resources, which are rival in consumption but non-excludable.