Which of the following is the best example of a 'Public Good' as defined in Public Economics?

A pure public good has two key characteristics: 1. Non-rivalry: One person's consumption does not reduce the amount available for others. 2. Non-excludability: It is impossible or prohibitively costly to prevent someone from enjoying the good. National defense is the classic example: protecting the country for one citizen does not diminish protection for others (non-rival), and you cannot exclude any citizen within the borders from its benefits (non-excludable). A toll highway is excludable (via tolls). A private college is both rival and excludable. A park with a fee is excludable.