I'm giving a pictorial representation (which I interpret), then it'll be easy to illustrate
First, cyclist sees $19$ miles on one side & $21$ miles to the other side.
Now, as soon as he approaches another milestone he sees that the next milestone read ‘$18$’ as he approached it and ‘$22$’ on the reverse
Whereas, he thinks that the milestones would read ‘$20$’ and ‘$20$’ on both sides.
The most suitable option click on the mind by seeing the illustration that the milestone on his side facing indicates the remaining distance to be covered by him to reach the destination & the reverse side indicates the total distance covered by him until now.
Hence, option B) is correct.