Which of the following is an example of a public good?

A public good is characterized by two main features: it is non-excludable, meaning that individuals cannot be effectively excluded from using it, and it is non-rivalrous, which means that one person's use of the good does not diminish another person's ability to use it. A clean environment exemplifies a public good, as everyone benefits from its existence without being excluded from its advantages. For instance, clean air and clear water support the health and well-being of the entire community simultaneously.

In contrast, private parks, advertising campaigns, and personal security services do not share these characteristics. Private parks typically charge admission, thus excluding those who do not pay, and they are limited in space, making them rivalrous. Advertising campaigns are generally aimed at promoting specific products or services, benefiting only those who pay for them, and do not provide universal public benefits. Personal security services protect only those individuals who pay for them, which makes these services excludable and rivalrous. Therefore, a clean environment is the only option that fits the definition of a public good.