Alright. So the question basically states everything for you. p is less than q. So just make up numbers that satisfy this. Lets say p=3 and q=7. And r=2. Just plug in the numbers now. p+r=5 and q+r=9.
A) says r-q is how much more less p+r is than q+r. When you do r-q, you get -5. But the difference between p+r and q+r isn't -5.
B) says that q+p is how much more less p+r is than q+r. But when you do q+p, you get 10, which isn't the difference.
C) says q-p is how much more less p+r is than q+r. When you plug in the numbers, for q-p, you get 4, which makes sense (correct answer).
Now, since you already know the answer's C, there's no real point in trying out answer choice D. Hope this helped!