The correct answer is B) ad valorem. Ad valorem is the term used to describe customs duties that are levied as a percentage of the estimated value of the goods being imported. This means the duty amount is proportional to the value of the merchandise, rather than a fixed amount per unit (specific duty). Ad valorem duties allow the government to collect revenue based on the actual worth of the imported goods, rather than a pre-determined rate. The other answer choices are incorrect - "secure" is not a term used for customs duties, "infinite" does not accurately describe ad valorem duties, and "specific" refers to a different type of customs duty where the rate is fixed per unit rather than based on value.