Final answer:
The true opportunity cost for the Johnsonville city council's decision to buy new garbage trucks is the forgone improvements to the sewage system.
Step-by-step explanation:
The concept of opportunity cost is pivotal in economics and refers to the cost of one item being the lost opportunity to do or consume something else. In the scenario provided, the Johnsonville city council faced a choice between making improvements to the sewage system and buying new garbage trucks. They chose to purchase new garbage trucks, which means the opportunity cost of this decision is the improvements to the sewage system they had to forgo. Therefore, the correct statement in this case is: D) In this situation, the opportunity cost was not being able to improve the sewage system.