Which of the following is a public good?

• A. A sandwich
• B. A haircut
• C. National defense
• D. An online streaming subscription

Answer: (C)

Public goods are non-excludable and non-rival in consumption. National defense fits this because once security is provided, you cannot realistically bar people from being protected, and one person’s protection doesn’t reduce the protection available to others. That combination—no easy way to exclude anyone and no depletion of value as more people benefit—defines a public good.

A sandwich, a haircut, and an online streaming subscription all fail on one or both of those features. A sandwich is excludable and rival: if you take it, others can’t have it, and the seller can prevent access. A haircut is also excludable and rival for the same reason. An online streaming service is excludable (you need a subscription to access), and while viewing may not be easily diminished by one additional viewer, the key point is that access can be restricted, so it’s not a non-excludable public good.