To determine the opportunity cost of Sergei's best decision, we first need to identify the option that makes him the most money.
The earning amounts for the options are as follows:
- Rake leaves: $50
- Make deliveries: $70
- Babysit: $65
The highest earning option is making deliveries for $70.
The opportunity cost is the value of the next best alternative that he is giving up by making this choice. The next best alternative after making deliveries is babysitting, which earns $65.
Therefore, the opportunity cost of the best decision (making deliveries for $70) is $65.
However, looking at the provided responses, it seems there was a misunderstanding in the responses as they do not include $65. The closest matches were repeated. Nevertheless, the correct answer to the opportunity cost is indeed $65.