C. Approximately 16 gallons of fuel.
First, multiply the average speed of 70 miles/hour times 4 hours, the number of hours traveled, to determine the total number of miles traveled in the time period, or 280 miles.
Next, divide the 280 miles traveled by the average fuel efficiency of 18 miles per gallon to determine the total number of gallons used during the 4-hour trip. You get 15.5666 gallons, or approximately 16 gallons of fuel.