Answer: the $10 he could have earned at his job.
Step-by-step explanation:
Opportunity cost is what an economic agent such as a individual and a firm forgoes when a choice is being made out of several options available.
In this case, we are told that Ali decides to attend the one-hour review session for microeconomics instead of working at his job and that his job pays him $10 per hour.
Ali's opportunity cost of attending the review session is the $10 he could have earned at his job.