The correct answer is C. The other goods that could have been produced with the resources used to build the school.
Opportunity cost is the cost of something in terms of the next best alternative that could have been chosen. In the case of building a new school, the opportunity cost is the value of the goods and services that could have been produced with the resources used to build the school. These resources could have been used to build a hospital, a road, or a park.
The other options are not the best opportunity cost because they do not represent the value of the resources that were used to build the school. Option A, the increased taxation to pay for the school, is simply a transfer of money from taxpayers to the school. Option B, the money that was spent on building the school, is the cost of the school, not the opportunity cost. Option D, the running cost of the school when it is opened, is the cost of operating the school, not the opportunity cost.
In conclusion, the best opportunity cost to society of building a new school is the value of the goods and services that could have been produced with the resources used to build the school.