The correct answer is "average speed."
Average speed refers to the total distance traveled divided by the total time taken, while velocity is a vector quantity that includes both speed and direction. Since the person jogs on a curvy trail, their direction changes, and therefore they cannot define a specific velocity (unless we consider the displacement from start to endpoint). Average velocity would typically represent the total displacement divided by total time, which is not specified here. Hence, the best answer is average speed at 6 miles/hour.