To determine the opportunity cost of the best decision, we first need to identify which job Sergei should choose based on the potential earnings.
- Raking leaves: $50
- Making deliveries: $70
- Babysitting: $65
The highest earning option is making deliveries for $70.
The opportunity cost is the value of the next best alternative that Sergei gives up by making his choice. In this case, the next best option after making deliveries is babysitting for $65.
Thus, the opportunity cost of choosing to make deliveries for $70 is $65.
Therefore, the correct response is $65.