Opportunity cost is the value or benefit given up by engaging in one activity relative to engaging in an alternative activity. It is chosing one activity giving up the opportunity to do a different option. So, the best one is chosen ignoring the less important thing. That is why opportunity cost is the best forgone alternative.
Example:
1.I have decided to rest in my free time in stead of wasting time gossiping with others for my fresh mind.
2. I have chosen to have regular exercise in stead of lazy time for better health.
3. I prefer to arise in the early morning for study in stead of late night for my health.