The answer to this is letter B. How was this conclusion reached? You need to check out the numbers used in the given as well as the direction used. According to the question, you are trying to get your dog to pull towards the direction that you want.
Since your dog pulled you 15 Newtons to the left and you tried to take control by doing 5 Newtons to the right, you can just subtract the numbers. 15-5 is equal to 10. Next, you need to check the direction. The rope is probably still leaning to the right since you were unable to pull it all the way to the left. This means that the answer to this question is 10 Newtons to the right which is letter B.