Answer: c. the cost of driving the next 15 miles, but not the cost of driving the first 800.
Explanation:
When calculating your OPPORTUNITY COST i.e your cost of not simply continuing on your journey in this instance, the relevant calculation is to calculate the cost of the next 15 miles to that previously unknown attraction.
This is because the 800 miles had already been accounted for in your journey to your initial destination and need not be double counted.
However, as you are now deviating from the plan for a 15 miles trip, that should be included in your existing opportunity cost calculation which already includes the original 800 miles.
