When measuring an earth magnitude, the Richter Scale is used. This is based on a 10 logarithmic scale. This means if an earthquake was a magnitude of 6 than it would be 10 times greater than a magnitude 5, so in your case if there were a earthquake a magnitude of 7 than it would be a 100 times stronger than a magnitude 5. So, the correct answer I believe is C.
Hope this helped!
