Final answer:
An ad valorem duty is a tariff, not a nontariff barrier; it is calculated based on the value of imported goods and does not fall under NTBs.
Step-by-step explanation:
A nontariff barrier (NTB) includes various regulations and policies that a country can employ to make importing goods more difficult or expensive, without imposing traditional tariffs. Examples of NTBs include quotas, discriminatory procurement policies, restrictive administrative and technical regulations, and rules-of-origin regulations. However, the one option listed in the question that is not considered a nontariff barrier is an ad valorem duty, which is actually a type of tariff that is calculated as a percentage of the value of the imported goods.