- The climbers fearlessly climbed on the mountain.
- On the mountain fearlessly climbed the climbers.
- Fearlessly climbed the climbers on the mountain.
- On the mountain climbed fearlessly the climbers.
The correct sentence structure in English is Place-Subject-Adverb-Verb. The place (on the mountain) should come first, followed by the subject (the climbers), the adverb (fearlessly), and finally, the verb (climbed). The correct option (a) follows this structure.