Answer:
The best sentence is: "A. Running on a treadmill is much easier than running outdoors".
Step-by-step explanation:
Letter A is the best answer because it contrasts "running on a treadmill" with "running outdoors" by the use of the expression "much easier than", which is a comparative and contrast pattern. Letter B is not correct because it is not a contrast, although it presents a relation between "running on a treadmill, like running outdoors". Letter C is also not correct for it is not a contrast, not even a comparison, it is only a statement. Then, letter D is also not a contrast, for it only presents a statement about whether doing something or something else.