Answer: $75
Step-by-step explanation:
The opportunity cost is the amount that you stand to lose if you do not take the next best alternative to your current course of action.
In this scenario that next best alternative is selling the ticket for $75 so this is the opportunity cost.
The $50 that was spent to buy the ticket is a sunk cost which means that it has already been incurred so it will not be accounted for in the opportunity cost analysis.