Answer: (a) specialization
Step-by-step explanation: out of all the four options (i.e.: specialization, price ceilings, scarcity, government regulation) specialization is the economic procedure that encourages voluntary exchange the most. This is because in specialization, a country is not self-sufficient; it does not produce everything itself. It only produces those good or services, which it produces best in terms of price and efficiency and hence, since it is only producing limited products, it will have to rely on trade to obtain goods and services that other country, are producing better.