Goods that are rival in consumption but not excludable are considered "common resources." So, the correct answer is:
b. common resources.
What is Common Resources?
Common resources are goods that are rival in consumption, which means that when one person consumes the good, it reduces the amount available for others to consume. This rivalry arises because there is competition for the use of the resource. However, common resources are not excludable, meaning it is difficult to exclude individuals from using the resource, and they are often available for use by anyone.
The classic example of common resources is a "commons" or a pasture where multiple farmers can graze their animals. Each farmer's use of the pasture (consumption) reduces the available grass for others, making it rival. However, it's challenging to exclude other farmers from using the pasture, which makes it non-excludable.
This is in contrast to private goods, which are both rivalrous and excludable. Club goods are excludable but not rivalrous, and public goods are neither rivalrous nor excludable.