Answer:
c. the cost of driving the next 25 miles, but not the cost of driving the first 500.
Explanation:
500 miles already have been driven and all the cost incurred before is considered to be sunk cost for the decision to be made. Any additional cost to change the decision or make the decision will be the opportunity cost of that event. In this example only 25 miles cost will be an opportunity cost of visiting the attraction never been visited before.