Final answer:
The action of putting a limit on the number of imported cars is known as a B. quota, which is a protectionist trade measure that imposes numerical limitations on the quantity of imports.
Step-by-step explanation:
The measure to put a limit on the number of cars that can be imported from other countries in an effort to help U.S. car companies sell more cars is an example of a quota. An import quota is one method of protectionism, which is a government's action to restrict international trade to help its own country's industries. Quotas impose numerical limitations on the quantity of products that a country can import which can lead to higher prices for those goods in the domestic market but can also benefit domestic producers by reducing foreign competition.