In this question, we can use the triangle inequality theorem which states that the sum of any 2 sides of a triangle must be greater than the measure of the third side.
The sum of the first leg and second leg of the triangular course is 9 + 7 = 16 miles
Then the third leg must be less than 16 miles.
We can rule out option A, because the measure must be less, not equal, and option D because a measure of 18 miles is greater than 16 and does not fulfill the theorem.
Option C does not seem reasonable, so Option B 14 miles is the one that fits best to be the measure of the last leg.
Answer: Option B. 14 miles