The correct answer is b) import quota.
An import quota is a type of trade restriction that sets a physical limit on the quantity of a particular good that can be imported into a country over a specified time period. By establishing a cap on imports, quotas are designed to protect domestic industries from foreign competition, promote local production, and stabilize prices.
In contrast, an import tariff refers to a tax imposed on imports, which raises the cost of foreign goods but does not directly limit their quantity. ‘Fixed favoritism’ and ‘voluntary’ (likely referring to voluntary export restraints) are not mechanisms that primarily restrict the quantity of imports in the way that an import quota does.