Milepost

If the miles on stone are in decreasing order, then these must be the miles to dest...decrease as approach...same is true on the other side of the mile post.

imo C

If the milepost gives the miles to the destination then as the hiker progresses the facing numbers would keep reducing in value. Hence from "21" it reduces to "20" when she completes a mile. Hence C is right.

C is crying to be picked up!

This is a "Resolve Paradox" type question.

Reading the stimulus one thing is clear, the total distance is 21 + 23 = 44 miles. The solution can almost be prephrased:
1. The stones facing her are going to increase in number - in this case they represent miles covered
2. The stones facing her are going to decrease in number - in this case they represent miles to go
3. There is an issue with the numbers on the milepost. the chances of this are almost nil since the addition of the values given in the stimulus is 20 + 24 = 44.

Hence let's look at the choices:
A - Even if they were reversed they should still show 22/22 and not 20/24 or 24/20. Hence out.
B - Does not matter what the quantity the logic will remain same for Kilometers, miles, yards .......Hence out
C - Spot on this is point 2 above. Keep.
D - Tough to justify, these are mileposts if one were missing then this one should have read 19 (facing)/25 or 23/21 (the sum has to be 44), depending if the posts represent distance to go or distance from start, respectively.
E - Bikers, hunters .....hikers I believe all use the same mile. A sailor should be able to figure out the logic even though he would be thinking in terms of nautical miles. Discard.

Hence pick C