Jon currently skates three miles a day, two days a week at a skating rink. how can he increase the intensity of his skating? 1.start skating five days a week 2.start skating in hilly areas 3.switch to skating four miles each day 4.add 10 minutes to each skating session

I would go with B(Start Skating In Hilly Areas).
If you think through the other options, A, C, and D don't really add up.
Option A would change the frequency.
Option C would change the distance.
Option D would change the duration.
So that leaves Option B which would make skating more difficult, hence increasing the intensity. 
Hope this helps :-)