Final answer:
The item that is not a public good is the private car, as it is both excludable and rivalrous, unlike clean air, national defense, and education, which are typically nonexcludable and non-rivalrous, making them public goods.
Step-by-step explanation:
The question asks which of the following is not a public good: 1) Education 2) Clean air 3) Private car 4) National defense.
A public good is defined by two main characteristics: nonexcludable and non-rivalrous. Nonexcludable means it's impossible or too costly to prevent anyone from using the good, and non-rivalrous means one person's use of the good doesn't reduce its availability to others. Considering these characteristics, the answer is 3) Private car. Private cars are excludable because the owner can control who uses it, and they are rivalrous because one person's use of the car prevents others from using it at the same time.
On the other hand, options such as clean air, national defense, and education are typically considered public goods. Clean air is nonexcludable and non-rivalrous as it is difficult to stop people from breathing the air around them, and one person's use doesn't diminish the availability for others. National defense is provided to an entire nation without the ability to exclude individuals, and its protection extends to all citizens regardless of individual contribution. Education, particularly primary and secondary education provided by the government, is often considered a public good due to its accessibility and non-rivalrous nature although it can have aspects of excludability and rivalry depending on how it's provided.