The correct answer is option 4 - 1, 2 and 3.

1. Absolute cost difference: This theory states that countries will engage in international trade if they can produce a particular good at a lower absolute cost compared to other countries. In other words, if a country has a natural advantage or resources that make it more efficient at producing a certain product, it will specialize in producing that product and export it.

2. Comparative cost difference: This theory expands on the concept of absolute cost difference and focuses on the relative efficiency of producing goods. It states that countries should specialize in producing goods in which they have a comparative advantage, even if they are not the most efficient producers of that particular good.

3. Opportunity cost: Opportunity cost refers to the potential loss of other alternatives when choosing to produce or consume a particular good. Countries will engage in international trade if they can benefit from producing goods with lower opportunity costs.

These three theories together form the basis of international trade. They explain why countries choose to engage in the exchange of goods and services across borders, based on differences in cost, efficiency, and opportunity cost.