Answer:
work out at the gym
Step-by-step explanation:
Opportunity cost may be explained as a situation where a positive loss is made in selecting one alternative from the list of other alternatives.
Here it is given the list of activities in a serial order :
1. go to the library
2. work out at the gym
3. have breakfast with friends
4. sleep late
So out of these four options or alternatives, one activity has to be chosen. And we decide to go the library as an activity to be accomplish.
By doing this we loose a potential gain from the other alternatives by choosing a certain option. That is by going to the library we lost the opportunity of going to the gym and doing exercise which is beneficial for our health.
Thus, the answer is work out at the gym.