Explanation:

Public goods are goods that are non-excludable and non-rivalrous in nature, meaning that they are available to everyone and one person's use of the good does not diminish its availability for others.

Among the options provided, option 'C' (Cars) is not an example of a public good. Here's why:

Roads:
- Roads are an example of a public good because they are available for everyone to use and one person's use of the road does not diminish its availability for others.
- Roads are non-excludable, as it is difficult to prevent someone from using a road.
- They are also non-rivalrous because one person's use of the road does not reduce its availability for others.

National defense:
- National defense is another example of a public good.
- It is non-excludable because it is difficult to exclude someone from benefiting from national defense.
- It is also non-rivalrous because one person's protection does not reduce the protection available to others.

National Forests:
- National Forests are also an example of a public good.
- They are non-excludable as they are open for everyone to visit and enjoy.
- They are also non-rivalrous because one person's enjoyment of the forest does not reduce its availability for others.

Cars:
- Cars, on the other hand, are not an example of a public good.
- They are excludable because ownership and use of a car can be restricted to the owner.
- Cars are also rivalrous because one person's use of a car reduces its availability for others.

In conclusion, cars are excludable and rivalrous, which makes them different from the characteristics of public goods. Public goods are non-excludable and non-rivalrous, and examples of public goods include national defense, roads, and national forests.