Trade takes place because of comparative advantage. Comparative advantage is a term used in economics to describe a situation where one country or person can produce a good or service at a lower opportunity cost than another country or person. In other words, it is more efficient for one country or person to produce a specific good or service, while another country or person specializes in producing a different good or service.
To understand this simply, consider a scenario where one country can produce cars more efficiently than another country, while the second country can produce computers more efficiently than the first country. If these two countries specialize in producing the goods that they are relatively more efficient at producing, and then trade with each other, both countries can benefit from the trade. This is because each country can obtain the goods they need at a lower cost than if they tried to produce everything themselves.
Therefore, the correct answer is comparative advantage. It is not marginal utility, inefficiency in production, or opportunity cost.