The correct answer is - A) a quota.
A quota is defined as the limited quantity of a particular product which under official controls can be produced, exported, or imported. With the policy that China has about the foreign movies, only 24 being allowed to be shown in a given year, it demonstrates a perfect example of a quota.
China is a country where the communism elements are still strong, and it is not only the movies of foreign origin that are restricted, but quotas are applied for numerous things, especially things connected with culture. In this manner, the politicians try to limit the foreign influence on their population, thus managing to preserve their ideologies and keep the society close to its indigenous culture and views.