"David attempts to minimize the costs of his relationship with Daniel while maximizing benefits. He enjoys Daniel’s company, but he tries to avoid helping him move. This is an example of the theory."

social exchange                  

Explanation:

Social exchange theory: In psychology, the term "social exchange theory" is described as a process whereby social behavior is said to be the consequence of a specific exchange process. The given exchange process aims to minimize the costs and therefore to maximize benefits.

The social exchange theory was introduced by one of the sociologists named George Homans.

In the question above, the given statement is an example of the social exchange theory".              

b) to encourage free trade