Answer :
The best answer here is isolated. If we use context clues, we can get an idea of what the author most likely means with the word secluded. They state that they ride in the "quiet and secluded French countryside." Knowing that it's quiet implies that it is most likely not a busy place with a lot of people. Bearing that in mind, secluded would have to be the best answer here.
Beautiful might make sense, but it doesn't match well with quiet and neither does complicated. If it was complicated, they would phrase the sentence differently. The same goes for mountainous as they don't imply anything related to that word.
Beautiful might make sense, but it doesn't match well with quiet and neither does complicated. If it was complicated, they would phrase the sentence differently. The same goes for mountainous as they don't imply anything related to that word.