The answer is D-8.
If Winnie walks 2 miles in 30 minutes, 30 minutes is half an hour so for 1 hour which is 60 minutes, she can walk 4 miles, 90 minutes she can walk 6 and for 120 minutes which is 2 hours she can walk 8.
So all I did was multiply 2 by 4 and got 8. The number 4 came from the thought that 2 hours have 4 sets of 30 minutes