Answer: A.
Explanation:
By definition, opportunity cost is the amount or value of something you gave up for another good.
For example: say you value sleeping in at $5 value going to class at $4. You decide to get up and go to class, the $4 value. Therefore, your opportunity cost is what you gave up (sleeping in) for another good/choice (going to class), is $5 since you valued sleeping in at that.