Concept:Exchange control refers to government policies that restrict or regulate the flow of currency and payments across international borders.Explanation:It is mainly used by governments to manage the external value of their domestic currency.By controlling the buying and selling of foreign exchange, the government can influence imports and exports.This directly affects the inflow and outflow of goods, services, and capital between countries.Therefore, exchange control is a tool applied to regulate foreign trade, not trade within a country or activities like barter or stock exchange.Answer:C. foreign trade