A hypothetical economy has a piece of land that, in order of priority, can be used for building a school, a hospital, a factory, and a residential complex. What is the opportunity cost of choosing the best alternative?

A hospital   

The “Opportunity cost”  of a resource basically means the value of the next-highest-valued alternative use of that resource. According to the question, if a person chooses the best alternative that means he built a school on it. Thus, in that case, the opportunity cost or the next best alternative for him will be a hospital.